I have no advice I'm afraid, but just wanted to empathise that this is one of my real fears. My DM is at an in between state at the moment - she has MCI (possibly tipping into early dementia) and is anxious and depressed, but she doesn't yet need care of any sort. However, if dad dropped dead tomorrow, then there's absolutely no chance that she'd be able to live alone. There's no sign that he will, as he's pretty fit and active - but he's an 80 year old with high blood pressure, so you really never know. I work FT and have two kids and live an hour and a half away, while DSis is the same and lives five hours away.
I don't really know how you prepare for the situation that you've found yourself in. I've thought of starting to look into assisted living options just in case, but don't know whether that should be near me or near mum - and given that mum has capacity for now, it feels deceitful to do that behind her back. But if I introduced the idea to her, she would totally freak out. And it might be a total waste of time - dad might well outlive her, and if he did die or become unable to look after her, it might be that she'd declined by then and would need a care home rather than assisted living.
I'm so sorry OP, I totally understand your panic, tbh. DM always told me to never ever consider having her living with me (she had the same with her own DGM), but I think she might feel differently now.
Does your MIL have money available? One thing I have found (through experience with FIL) is that it's relatively easy to find somewhere for respite care at quite short notice, so I wonder whether the same might be true of supported living? Might there be an option for a temporary stay in a nice supported living place, with a view to it becoming permanent if your MIL likes it? Does supported living have to be bought, or is it something you can rent (and would your MIL be able to afford that while the house is being sold?). Has your MIL given voice to any thoughts about long term plans, or is everything still too raw? And how's her own health and cognition?