I've had slightly similar although to a lesser extent (and my MIL did buy a highchair!). MIL used to complain that BIL and his wife moved her stuff when the children were small, so we didn't feel we could move anything (DS1 was 18 months when we moved from NZ so hadn't visited before that).
I tried to get myself as far away from DS1 as possible (if that's difficult you could always lie down with a headache for an hour), so that it couldn't be seen as my responsibility, and then when he started approaching the ornaments she and DH would be trying to move him, and after a while she realised that it was just impossible and her things were going to get broken. So in the end it was her decision to move them.
I would also say to your DH that you understand why she doesn't want to move her stuff (I think it must feel a bit invasive), but that you find it really stressful having to mind DS in that situation, and that since it's his parents it would help if he could take more responsibility for monitoring your son.
Safety stuff is another tricky issue. Fortunately I do have a good relationship with my MIL and I explained that I was really freaked out by the possibility of DS1 getting into cleaning products and medicines, and she did agree to put some of the products up higher though I never had much luck with the kitchen (I bought a lock but she didn't really use it, and her and DH's view was that there were always people around, which was probably true, so I let it go but always asked someone to be personally responsible for DS1 if I left the area).
You could always ask if she would mind if you bought a (cheap) highchair and stored it there, as it would make things so much easier when you stay (and say that you really enjoy staying there but find it stressful without the baby equipment). Or get your DH to ask if you think that would work better.
Good luck!