And yes, from the different approaches I find the limit on reg numbers the best. There would be no errata and thats great.
The more I think about it, the more I like the idea of limiting any unit (regular or elite) to 2 ...
The only deployment cards, that are played more than twice in any list I've seen so far, are those units, which are overpowered (especially Royal Guards and Imperial Officers).
I still think, that the Royal Guards are overpowered. And this wouldn't change by limiting them to 2.
I think this is a crucial piece. From what I have tried and seen of the game, it would make a lot of sense to cap non-Elites to two copies as well, BUT that has little to do with the Royal Guard in fact. You'd probably still see people play 2xRG in every Imperial list, because, as has been pointed out by DerBaer in his other topic (evaluating the different imperial deployment cards) RGs are over the curve in every aspect.
Without having the full overview of things, my gamer-stomach tells me that most of the other units are reasonably balanced among eachother with 'perhaps' the Sabs being on the stronger end too, but not too strong because they can still easily die if the White die doesn't favor you and without Focus they lose a lot of strength.
So a solution could absolutely be to both limit non-elites to two, issue a new Skirmish version of the Royal Guards (and perhaps on top hire some competitive playtesters for Skirmish along with getting some stat-guidelines rocking it).