I am sorry that I can't offer any help, just a similar experience.
When MIL was in her first CH she could never find the toilet down the corridor. The staff got cross with her wandering around at night so they thought the solution was to put a commode in her room. She would get the bowl out and try to tip the contents out of the window. Since there was a security limiter on the window it would only open 4 inches. Thus the contents of the pot were spilled inside the UPV window frame, inside the radiator, down the wall, and on the carpet. She also hid pieces of poo in a vase under her bed.
We were livid at their poor management of her toileting, and the resulting embarrassment, loss of dignity, and hideous pong. In a CH, where night staff are employed, there is no excuse for lack of care at night.
What to do when you don't have carers on night duty? I just don't know. It sounds as if your MIL needs night-time toileting supervision. I can't see how you can stop her trying to empty the commode if she thinks it is dirty to leave her wee in the bowl.
This sounds a difficult problem to solve, 1954. I can only suggest you aim for damage limitation by putting a substantial waterproof material down on the floor eg. a double bed size Kylie. Polythene or plastic could be too slippery underfoot. This idea would at least limit the damage, but could mean a lot of washing and drying for you. Sorry I can't think of any way to prevent the dismantling.
Same with mum when commode was first put into her bedroom, she would use it and then remove pot to take to toilet. Tried tape at first, but the only thing that worked was supervision, as my bedroom next door to hers I could hear every movement and once I started waking her around midnight to use commode I found that we had no accidents and if she uses commode after that, the pot stays exactly where it is until I clear it.
Thank you for responses. At least I know I am not on my own. Thankfully we put Lino on the floor before we moved her into the in anticipation of this problem. We have floorboards so they would have got ruined. Will try large pads under commode