Ok. Finally on the laptop!
First off, I know only too well the effects of dementia, having worked with elderly people with the disease and my grandad showing signs too. UTIs are so commons its unreal! Your grandad needs to see he has a problem, he might be stubborn and it might take time sadly but unless he sees there is a issue its hard to deal with which makes family members feel frustrated and helpless. Which I completely get. Though going to his gp may be the wake up call he needs. After that medication doesn't cure the disease but it can prevent it becoming worse or slow the disease down. In terms of self help things my grandad writes lists ,incase he forgets.With your mum she is most likely worried about her dad and herself and getting anger as she is stressed and doesn't understand dementia.
I would try the firm but fair way, you say she discharges herself. Give her reasons why she shouldn't. Against medical advice for example, which sometimes peple dont care about, they have it in their head there going and that's that! Maybe getting the doctor or nurse t actually explain the op she has had and how big of a op this is, could help. And for you Ale, tell yourself you are doing All that you can, Youre being there, your are being supportive. You will be helping them by being there. Just keep going been you, it sounds silly. But sometimes that's all is needed or you can do. Not to beat yourself up, you dont need or desvere that!