Ok, now for the long drawn out post I'm making about all those points EC has brought up. I'm using my precious and limited painting time to do this so be kind

I'm going to put this into two parts. The first is points from the story in the supplement that relate to the skitarii. The second will be how those points translate into what we need to do with the list.
1. There are three distinct AdMech forces at the onset of the ork invasion; conscript militia, the skitarii garrison, and the knights.
2. The relief force sent by the Forgeworld was nearly the same size of the skitarii garrison.
3. Accounting for the 200k skitarii from the garrison locked in port Alpha, the Skitarii in the relief force accounted for 2/3 of the Skitarii during the final battles for Complex One and Port Omega. (this is higher if you include skitarii armored regiments and logistics regiments from the relief force)
4. Most OOB relates to the Skitarii either a. retreating or b. being slaughtered. This doesn't lend itself very well to determining how they fight offensively or defensively.
5. The only defensive installation mentioned is the Defense Laser. (walls aren't installations anymore than cliffs as described and ICBM's or Support missiles are always described as firing from Minorus or titans rather than fixed locations)
6. There are quite a few minorus present and several Majoris.
Unless things have changed since the version I read was posted, there are three admech lists at the start of the supplement: Hab militia, Skitarii, and Knights. Later this is augmented by Armored Regiments and Titans. Of these, the Hab militia seems to fit best with what you'd call a traditional PDF. It's an army raised on planet that is the first line of defense and does little more than waste everyone's time before the serious fighting starts. Knights and Titans we don't really need to discuss. At this point we have two flavors of SKitarii. Since both are operating in regimental sized units I think that it's fair to have two distinct lists representing them. Armored I think is the easiest and there's not much reason to discuss them right now. So that leaves the skitarii. First off, I don't think you can label them as being a PDF by the traditional sense, especially since the majority of skitarii engaged are sent by the forgeworld. I believe a better name might be the "Skitarii Legions" for the skitarii list. This follows the naming convention of the Titan list and avoids semantics around the PDF term. It also allows us to call the armored list either the "Skitarii Armored Legion" or "Cataphractii Legion" which again unifies the the naming conventions.
Secondly, I think EC ran out of Skitarii around hour 5 of the final battle.

Third, while pivotal in the story plot for setting, the walls and defense lasers aren't really central enough to put in a list as units that you can take multiples of. The reasons for this are as follows. Walls. While these do take a prominent role in the final battle and are constructed by the AdMech, they are manned by both skitarii and IG so I can't use their existence as reason enough to include them and they work best as a fixed scenario item to ensure that the table matches the story itself. Last, the skitarii were primarily from off world which makes it hard to justify this as standard practice for their forces seeing it took over a year to build the walls. The Defense laser appears in a pivotal role in the battle for complex 1, but it is still just a single installation. I think this is a cool idea to add into the list though so I'm still proposing that it be allowed as an upgrade that allows an AdMech player to replace an objective on his side of the table with.
For the most part that is what I've absorbed from the story for the supplement. I may have missed something, but I'm not glossing over anything on purpose here. It's a pretty long story and I'm human. The second part of this is reconciling the information in the supplement with the rest of the information we have on skitarii (all precious little of it). I'll leave that for now. Last of all is the list of objections that EC has and I'll deal with that now.
A. Focus not on ordinatus.
I think that this is either a miscommunication on my part or a difference in definition as to what that is sufficient focus on ordinatus. From my viewpoint 50% of the core options are ordinatus while the other half is skitarii infantry. Outside of that, you have the upgrade option for Hypaspists to take a minorus which has been extended to the Sagitarii. This means that any infantry can take a minorus and really only Knights, Robots and Spacecraft don't get the capability. Added to that the majoris and I think that we have a pretty heavy focus on ordinatus despite the majoris moving to support. I can understand the limits to the Majoris being a sticking point, but I think we are all in agreement that additional types will be added to the existing three. This should provide sufficient focus on the ordinatus.
B. Focus not defensive.
Outside of stationary and slow moving elements I don't see how one differentiates between a defensive list and an offensive one. The core units in each are identical to the point that unless you force people to buy installations the lists possible would be practically the same. You'd have the same Hypaspists, Minorus, Majoris, Robots, Sagitarii, Praetorians. Perhaps minor adjustments like making praetorians their own detachment would help differentiate other doctrines but you effectively have the same thing. This makes me conclude it should be the players decision and not the lists. However, I'm willing to hear EC out if he wants to expand on what he sees making a list defensive.
C. Too Many Units.
I'm grouping this with the next several since they all tie into it in one way or another. I understand the concern and I'll do what I can where I can but I don't want to sacrifice the diversity if it can be helped. To that end we've stripped out the Russes and limited the Macharius to allies and restricted the majoris to named versions. All of this combines into a fairly decent pruning of units without touching titans.
D. Strip CLP.
I don't want to see the CLP go, but I'm willing to work out conditional pricing on it. With the limits preventing the Majoris from abusing the CLP minorus I think this just amounts to pricing it for the Quake cannons but we can discuss this more in depth in another thread so it doesn't get lost in the shuffle.
E.F. - I think we have already addressed this with Test list D.
J. K. - We have 12 weapons options for the minorus and majoris as it stands now. Of those some really aren't useful so I'd say we really only have about 8-10 options each that are viable. EC is proposing cutting the unit count by restricting the options here but were wanting about 10 set versions of each which basically leaves us where we are now with unit count. I think the better route is to restrict the Majoris and add several more options to cover gaps in style based on the fluff we have and leave the minorus alone outside of tweaks to the chassis cost if necessary and conditional pricing for the CLP.
I. Titans are one aspect that I believe is core to the functioning of the ADMech as a whole and I won't reduce or change the ability to customize them. If the Titans are balanced to their point cost they should also be pretty close as allies.
Ok backtracking:
G. See my previous comments on the supplement. I think the only installation that is worth adding from the fluff in the supplement is the defense laser. It adds a cool thematic element to the skitarii list and helps to facilitate defensive play style by inviting the player to garrison units near it since it would be pretty vulnerable without support.
H. See previous about the naming of the skitarii list.
Ordinatii (minorus): The faster speed isn't supposed to completely absolve the arty thing. It's supposed to make the minorus at least somewhat attractive in other rolls. Making them slower than molasses doesn't give them character it makes them useless for everything EXCEPT stationary rolls which in turn makes arty versions more desirable. Even if we eliminated arty versions entirely, direct fire ones wouldn't immediately appear since one of the core issues is they are too slow to do anything. The net result would likely be that the minorus just isn't taken unless needed as a transport. The 5cm enhances the theme by making the minorus more desirable on the table and therefore more likely to be taken with infantry in keeping with the descriptions from Dark Apostle while differentiating them from the majoris. There are other attributes of the Minorus that differentiates them from your regular SHT including shields, DC, weapons options and AA capability. With conditional pricing on the CLP, I think we'll see more direct fire minorus show up.
Outside of the CLP+quake being a good deal, the rest of the options except maybe the Corvus don't strike me as either too good or poor for their points. Besides, with the number of options you want for each, we might as well just spend the effort checking the current weapons since you are looking at just as many pieces to go over.
Last, I'm not sure why you want changes to the list construction style. It's not broken now. I don't think one way is better than the other, but it's more work and dubious benefit to the list.
So in summary:
a. focus not on ordinatus - List is already Ordinatus heavy and will gain more majoris types
b. focus not defensive - Define what makes a list defensive since it seems mostly up to the player.
c. too many units - Reductions have taken place. Majoris free selection, russes, limited macharius.
d. strip CLP - Will work on conditional pricing as suggested. Probable price increase of 25 points per quake cannon.
e. strip Russes - done
f. strip macharius - mostly done, but they should stay allies to fit fluff. GW/BL trumps fanfic. As allies you can also write them in as IG if you wish.
g. add installations - Defense laser is the only real installation mentioned and will attempt to add.
h. definition of PDF not a PDF - Should be resolved by changing the name of the list to fit both existing fluff and the supplement.
i. limit titans types - Won't happen as mentioned multiple times.
j. limit minorus types - Unsure of gain from this with proposal for expanded "fixed" types nearly equaling the current number.
k. limit majoris types - Already done
That's 6.5 out of 12 with further discussion needed on 2. If you want to make an argument for any of the others feel free to do so but this is what I've got based on what I read in the supplement fluff and what I believe to be most fitting of the official fluff as well.