Person A was completely correct to want the bigger space since she is the third wheel and can easily be outnumbered in the shared space, at least with the bigger space she can retreat if there is fighting and drama with the other two dating antics.
Since the other two B & C will end up dominating the shared space with work desks and just the 2 on 1 thing, person A is going to end up using common area it much less, like she'll never be able to entertain or watch TV during the day since that space will be occupied by a pair of work from home dating adults. Unless I'm missing something I almost think there should be nothing extra, but the $50 is ok to just keep the peace I guess, certainly not much more considering the probable outcome.
I wonder, whose name is on the lease? Do you know whose lap the responsibility will fall if there is a blowup? For both my kids they ended up signing leases in towns where the whole thing can land on one person so I wrote out my own contract to be signed by the roommates and their parents that said something like:
"I agree to pay ---- portion of rent until the end of the lease regardless of circumstances." I know too many people whose kids had roomates leave school and had to cough up unexpected $ because the landlords can't be bothered with expenses of chasing them down if the other roommates are on the hook. At least with this I could do small claims if needed, thank goodness I didn't need to but better safe than sorry.