OP, your ILs sound like a horror to me, especially MIL! She makes my difficult MIL of years ago look like a gentle dove! And you're expected to spend 10 days w/ her twice a year? Whoa! You have my sympathies.
'I feel like she is having a hard time letting go of the mother role and transitioning into grandmother."
I agree. And I would understand that except that she doesn't even seem to be trying. That's why, for example, I wouldn't use her cake (might put it in the freezer for another day). Two cakes are all right, IMO, but not if MIL is competing w/ mum, which clearly, your MIL is.
As for the gifts, I don't get the impression you mind them so much as the bragging and criticism that comes w/ them. I agree w/ PPs (previous posters) that you should accept the gifts graciously. But call a halt to the unnecessary comments about "ingredients." Let them know you and your kids aren't interested in that.
Overall, I agree w/ PPs who say you need to set boundaries. First, IMO, you need to decide in your own mind what limits you wish to set. Then talk it over w/ DH, you may find you have to make some compromises w/ him, but don't agree to anything that makes you unhappy. Finally, you two will have to present your boundaries to your ILs.
Will they have some limits of their own that they want to assert? Perhaps. But they are in YOUR home interacting w/ You and YOUR children, so you two get the final say.
You haven't said yet if it would be possible for them to stay in a hotel or a B & B. Since they seem to be well off, I don't see why not. In fact, that may have to be one of your boundaries - Yes, they can come and stay 10 days, but it has to be in a B & B, and they can only come over every other day or for X number of hours each day, etc. (you and DH may have to do some negotiating w/ each other on that one). Make other appointments if you have to, so that you and the kids simply won't be there at certain times, etc. Be kind and gracious, but be firm.
Good luck! And please let us know how things pan out.