Well, I can only speak of my own experience (and I'm not gonna pretend like I'm the absolute authority on the subject or anything), but having confidence in yourself - both sexually and personally - and having the will to be able to take control of another person during the act is sort of part and parcel with being the dominant in a dom/sub arrangement. At least a serious one; as in, not one in which the sub is telling the dominant how to act, simply because the usual dominant wants to be subbed for a change... which would probably be pretty unsatisfying.
Not if that's how it starts out, but getting the experience to play the dominant role convincingly is not the same as changing personality or acquiring new personality traits. Some people can rolaplay convincingly and then it just works, like the good Todd Howard would say.
In my experience it's just an individual question, some people can enjoy role playing and do it well, some don't, but playing the part does not have to mean abandoning your character assuming it's a private matter. Of course if you become a more generally dominant sexual partner that usually implies that the person too has changed, as it's no longe a role being played just with one specific partner. At least that's my theory.
I think that for monogamous relationships the only limits really are sexual openness and intimate comfort.
You can't be generally blushy and innocent of personality if you're a genuine dominant - it just doesn't work (unless, as I said earlier, it's an act). I'm not sure the exact time period that this game will take place over since I don't know its length yet, but going from nearly-no experience to be convincing as a dominant, especially if you have no real proclivity towards it in the first place (as it is with Holly), is really pushing the suspension of disbelieve.
Like you said yourself, it can be an act. That's how it is in my relationship for example. You wouldn't ever guess that my wife could play the dominant role convincingly if you met her, but she can.
Now, it's rare that she does play the dominant role, as it doesn't come to her naturally, as I imagine the same would be true of Holly, but when we role play certain roles she can fill the part.
Of course our sex life is far from traditional, even by lesbian standards, but then again a game like ORS should allow for such options, within reason of course. I would only want a dom Holly route if Eva think it could be fun and she can make it happen in a convincing manner.
That's why I struggle to see Holly truly fitting that role. I mean, I certainly don't begrudge the people who want it (because variety is nice), but in terms of authenticity and staying in character, unless it's really thought through and built well, it can easily feel contrived. Even moreso when Holly falls so effortlessly into the Sub category.
Then again, if I'm not playing it and it's what the people who are playing it want, I suppose it makes no real difference to me (I just find it weird).
No, you're right, it's not something that can just happen, but the ideal would be that it was a continous process, perhaps not even a succesfull one. Lena could ask the task of Holly and end up having to let it go because Holly can't get into it, that would be a fair development too (although I imagine such a turn of events would piss off those few that do like the idea of dom Holly).