Hello PaulaJaney,
Although there are practical things you have to do after someone dies, every task feels too huge to manage when you are grieving. I think, for some people, going at it at 100 miles an hour is their way of coping with the grief. They throw themselves into sorting all the practical stuff to avoid thinking about the more personal side of things. For others; for me; that didn't work. Much like you, I found it all too hard. I had no one to help me, so I had to force myself to do it, but I still resent all the additional stress it added to my weary shoulders. So, in a way, as long as you are comfortable with someone else doing it, let them get on with it. It will be a little less pressure on you. If you are uncomfortable with it. perhaps talk to her and try to get her to hold off or slow down with the things you are uncomfortable about.
Try to take care of yourself and maybe talk to your partner's mum. She may think she is helping you or just trying to block out her own grief. Either way, you probably both need to take some time to catch your breath. As long as financial matters are sorted and you have access to money, most things can wait a while. If tax is involved, you have a year to sort that out. Bills and deeds can be changed over in time, but as long as the death is registered, there is little else that is urgent, so take your time and let his mum get on with doing what she wants to do, but tell her gently, if you feel uncomfortable with anything she is doing. Be gentle with one another. You have enough to cope with at the moment. Thinking of you..