How about Naomi that we both accept that (a) we have thought about it, but (b) it's possible to disagree even after that? What I was particularly stunned by all the same, and remain so, is this insistence that there are only two outcomes that you (and Khandro) seem fixated on, ie stable Assad-led or unstable ISIS-dominated. Even if a third option, and I confess I don't have a particularly detailed one, is also unstable it doesn't have to mean that ISIS should be the big winner in that picture.
In answer to your more direct questions, (a) no idea, probably none of them (other factions); and (b) probably protest in violence, as always is the case whenever these sorts of things break down. The lesson I prefer to draw from history though is that violence only ever comes to end when people *try*. Success isn't guaranteed even then, but it's more likely to come about than just standing to one side.
I suppose if we are doing the "bad comparisons to Hitler" thing, is there not an analogy to be drawn in that way too? Even Hitler, bad as he was, was still preferred by some as a strong leader capable of keeping the "even worse" Stalin at bay from pushing deep into Europe. I am not sure that people would accept that this view was correct in hindsight. At any rate, being so scared then of hypothetically "even worse" scenarios just allowed actually real horror scenarios to happen anyway -- which is the only analogy I'm tempting to draw, and I hope there's rather less quarrel in this case than with Spicer's comments.
In the same way, I have every sympathy with being afraid of the consequences of intervention, if not properly followed-through and planned, in Syria. But so far our inaction has allowed a war to develop anyway in which hundreds of thousands have died, and approximately a quarter of the population has been forced to flee from their homes. Will an alternative be worse? Yes, it could be -- but it doesn't have to be.