Perhaps I'm being a little cold blooded here, but since she doesn't want to share a room with you anyway, and you are all paying your own way, why not allow her to stay anywhere she wants and you do the same? I strongly suspect that half the problem with travelling with relatives is getting caught up inthe notion that everyone has to agree about everything,instead of just doing what you want to do, and letting them do the same.
At least, this is the strategy I have. My SIL and her daughters are meeting us in WDW next weekend, and my plan is that if we want to do A and they want to do B, we shall do A, while they do B. I'll let you know if this works, or if it all ends in disaster.
Of course, if staying in different locations is really counter to the plans, and you can't get a WL code, you could try 2 rooms at an All Stars. No hot tub, but you're still in Disney. You probably can't get as cheap as HIFS, but you can probably justify it by the savings in parking (free), convenience (Extra Magic Hour, bus directly to Magic Kingdom gates, charge to room, package delivery to resort). Maybe if your mother only had to pay the same amount she would have paid at HIFS, she'd go along with having her own bathroom (providing this is financially feasible to you)?
Ciao,
Mort.