Any criticism below is intended only constructively - I'd like to see this list go Gold and get approved. I've played against it about half a dozen times, and played with it just once so have a decent handle on how it works.
It's always the Ordinatus is the sticking point for players; even after its reduction from BP10 Disrupt it remains the one unit in the list that seems to get called "cheesy" or generate negative feedback from opponents, and then require posts defending it.
I don't have an answer or suggestion for this right now, but when I have a moment I'll do some comparisons vs other super-artillery options and formations and see if it's actually fairly priced and statted vs its peers or help suggest something that make it palatable. Whatever happens, the final version shouldn't require the IW player to apologise for the damage before and after firing

Regarding the other suggestions:
Quote:
Allowing Dreadnoughts to take a 100pt Supreme Commander upgrade (think of Berossus)
- Allowing Stalkers to be taken as upgrades for the artillery formations
I like the sound of Berossus style commander. However, at the moment the Supreme Commander is the only reason a player is required to take infantry at all (correct me if I am wrong), and moving the AA Stalkers to Artillery Formations would reduce another reason to take infantry and transport formations.
If the result is the list
can be built with a SC and AA without any infantry, it inevitably
will be built without any infantry. IE this would have all the AP-ignoring strengths of Minervans, but combined with cheap core units for activation numbers and Macro CC spider-tank units.
I'm not going to say that doesn't sound fun, but it would be a real monster for oppponents to have to tackle.
Recommendation: ensure infantry are still required, even if not en masse.
Recommendation: Since Defilers are core in this army, make the formations larger instead of increasing their cost. 5 or 6 at the same price per model would be more useful in combat and shooting, harder to break, but put more of a limit on total activations as Core formations usually do.
Related question: Are defilers definitely the sort of thing that should be core?
Quote:
- Reducing the number of Obliterators in the Obliterator Cult to 4 with a cost reduction to 350
Good suggestion, but this actually makes oblits more appealing rather than less. The obliterator is an unusual case because each put out a remarkable amount of firepower (3x AT4+ when sustaining 45cm after teleporting each) so they will still do their job of disrupting deployment and killing counter-battery fire.
The smaller and cheaper the formation, the less risk to the player; each obliterator is enough to kill one and break a formation of deathstrikes, basilisks or hydras (causing average 1.25 kills on those models each).
TLDR: Smaller cheaper more numerous formations do not weaken the impact of oblits.
Quote:
I'm a bit constrained about changing stats/abilities on the Defiler and Obliterator as they are in use in other lists and have been approved.
There are precedents of units changing when they become formations rather than upgrades if you do decide you need to, although it usually needs a distinct name like Iron Obliterators. Defilers are perfectly fine, the only difference is making them work as a Core choice instead of a restricted one.
Oblits are phenomonal, one of the best units in the game even when not combined with the affect of Artillery on opponent's deployment. I've never been hit by it, but still think 15 AA attacks from a teleporting unit that isn't a dedicated AA unit might be about 10 too many.

Recommendation:
Obliterators change from 3xBody Weapons AP5/AT5/AA6
To
Iron Warrior Obliterators 3xBody Weapons AP5/AT5 AND Body Reapers AA6+
This doesn't weaken them in the vast majority of their intended uses but could stop the list hitting an approval road-bump when someone runs several thunderhawks into a Ordinatus protected by 10 oblits (30 AA shots+2 AA shots) and posts "lol whut??" on taccomm.

Hope that helps.
As mentioned, cricitism is intended to be constructive; if something is going to come up as a sticking point preventing approval I'd rather it comes up now than later!