Welcome to Dementia Support Forum
@runner bean.
It is difficult to say what to expect from your mother in law's diagnosis, though as it sounds as though vascular dementia is in the mix as well as Alzheimer's there may be times where she seems to get suddenly worse and then stays at a plateau, as vascular dementia tends to go in steps rather than in a gradual decline.
It's good you've got carers coming in, but it might be time to have a chat with them about making sure she is washing, eating etc. rather than taking her word for it. However she may not be allowing them to help her, and carers coming in are on a strict timetable and don't always have time to persuade the people they are visiting to change their clothes etc.
You also might be getting to the time when you need to consider either more care visits or a move to a care home. It might be worth starting to look at likely homes just in case. Certainly with my mother things were chugging along reasonably fine until suddenly they really weren't any more. I'd earmarked a possible care home and I was able to move her there quite quickly.
This is a very friendly and supportive place and I'm sure others will be along with their suggestions very shortly.