I can relate to this too with two of my elderly relatives. One of them developed a kind of dementia....vascular I think rather than Alzheimers and eventually died to everyone's great relief. The other is still alive and from time to time drives us all nuts. Not, I am glad to say to the extent that your MIL seems to be, but bad enough to make visits a trial.
The only way we found/find to deal with it is to try to ignore it as much as is humanly possible (very difficult). Go in, do the necessary and get out asap. Accepting that the lovely person you used to know has gone and been replaced by someone else using her body was the only way we could cope. It is just so sad too, though.
I have enormous sympathy for people who just snap and lash out at their elderly relatives whilst having this experience. Fortunately I never have, but it has been a close call sometimes. The frustration when you see them sabotaging arrangements or when you have to listen to them belittling all attempts to help them is just awful. We too have had the experience of them being as sweet as honey to outsiders, often to criticise them roundly as soon as they have left.
I have no idea what to do about this in terms of alerting the medical profession etc, but
the evidence is beginning to build up that yours is not an isolated case.
(Actually if you ave a look around Sainsburys or Asda and see an elderly person with their 60-ish companion, you can often see similar things happening. I certainly have.)
I have not contacted Alzheimers Society, but I think this might be the way to go for any advice.
I do feel for you .