While your W made allusions to things other than financial stress... you omitted she did not say "to work on the M".
So, IMHO:
IF you move back in, then it is for financial reasons. Your W will be a room mate. As she said, she'd be moving into the spare room.
Her moving into the spare room is her choice. She offered it. She did not suggest moving YOU into the spare room. Nuff said. Prolly no value mentioning to your W.
Without pressure, is there any way you feel you can communicate your concerns and any possible boundaries you might need, PRIOR to giving your landlord notice?