My heart goes out to you as you struggle with the overwhelming challenges you face regarding your mum.
The deterioration after each surgery leads me to ask whether your mum's B12 levels have been checked (not just B12 but Active B12 also). Personally, I would be insisting this be checked out, if it hasn't been already.
B12 levels may have nothing to do with the cognitive decline, although there is info out there regarding the impact of surgery/anesthetic (leading to cognitive decline) on patients who have pre-existing B12 deficiency. The more surgeries the more severe the deficiency becomes, if left unchecked.
Personally experienced this some years ago, in my late 30s/early 40s, after gallbladder surgery. Mentioned to a few doctors that I just didn't feel right, physically and mentally. Weeks passed before the deficiency became so significant that I could barely function. Blood test revealed severe deficiency. The B12/surgery link can often be overlooked in older patients especially as it can be regarded as 'normal' for there to be some impact after anesthetic. It's definitely frustrating when 'normal' to a health professional proves torturous to family members who are left to face the true everyday challenges.
I'm hoping this gives you an avenue to explore, in the way of making a difference. I know it may not help entirely but thought it worth sharing.
Take care and remember to treat yourself to some much needed support and stress relief.