Can we agree on some points ?
We talk about game rules.
You can only do things as part of the game or in the game which are written down explicitly in the rules. This writing down might be ambiguous and open for interpretation, but the concept has at least to be presented.
Actions and concepts not presented in the rules are not part of the game.
Army lists or army books might add or change rules and concepts for the armies they are written for.
If we agree on the above my train of thought would be (it is my tot anyway regardless if we agree on the above or not

)
The concept of not deploying all units of a formation or dropping units of formation on deployment is nowhere to be found in the rules. It is nowhere implied that you can change a formations composition at deployment. The chapter about garrison only states that certain requirements must be met by a formation to garrison, but it not even hints that you may drop units in order to make a formation meet this requirements.
This is also no common or universal game concept, neither in board games nor in tabletops. In tabletops the army composition is usually done before deployment. If your unit has to fulfill certain requirements it must do so when you "create" it.
The only reference to dropping units is in the SM army list. As this is an army list and no reference is made to other armies, there is no reason to suspect that this new concept (new, because it has not be presented before in the rules) could also be used with other armies.
As there is no concept for dropping units (except for SMs) for garrison requirements I do not know why it should be possible for any other reason - like cramming a formation into a WE.
And it doesn't matter whether Jervis might have intended to do this - it didn't make it into the rules.
That reminds me when infantry doesn't move - the troopers will usually lay down and try to make a target as small as possible. So my infantry has to be treated as being in cover when they didn't move. It's nowhere in the rules, makes sense and I would like to have it that way. That's about the same qualification as the dropping units rule.
Anyway - if whatever the procedure might be - it is decided to make a rule change and put this concept (of dropping units at deployment and not getting cover for not moving) in word into the rules, I'm happy with it. But without that written addition to the rules, I think it's not rules conform - because it's just not in the rules.
Regards
SH