I don’t know whether you’ve ever been there, Awesomegranny, but believe me, dementia can be unbelievably stressful to live with. My FiL (who I was extremely fond of) developed vascular dementia in his 80s, and we took him to live with us, blithely thinking, ‘Just getting a bit more forgetful - how hard can that be?’
My goodness, it wasn’t long before we found out. Within a year he was in a care home - 95% of it had fallen on me and I was at breaking point.
So I don’t think it unreasonable of the partner not to want to take on full time carer duties, but nor do I think the arrangement you suggest is at all unreasonable, either. However, given that her own property is rented out, she is highly unlikely to be able to move back at short notice.
Please be aware that live-in care can work out even more expensive than a nice care home - we looked into it once for an aunt of dh - especially if a carer is likely to have frequent broken nights, and/or the person is no longer safe to be left alone at all -not uncommon with dementia. In such cases there would need to be 2 or 3 on shifts, to allow for enough sleep and breaks/time off.
Plus you have all the costs of running a house, food etc. on top.