It sounds like your grandmother needs full time medical care, in a nursing home, whether in the UK or Spain.
My mother was like this with dementia and it was so grim till GP said she needed medical care and she went from hospital to a nursing home.
I'm so sorry for your mum and uncle having to deal with this - they really can't provide the level of care she needs, and also, unless they have medical training, won't be able too. My mums carers in the nursing home would change her pads, even if it took 3 of them, as it's non-negotiable. When it was just me, if she got violent I just couldn't get it done.
Your grandfather can visit your grandmother in a nursing home every day, so please don't let him missing her be a reason to put off full time care any longer.
Interim advice is to get some paid care in, as soon as you can, it's not a single handed job, as your mum knows.
As for tip for the time being- for meds, we used to grind them and put them a sandwich with whiskey marmalade, covered the taste.
For soling the bed - use the absorbant oversheets for kids who wet the bed or puppy traing pads over the sheets (matress protector underneath) and you might only have to change the protective cover and not strip the bed.
To stop her wandering - new bolts on external doors, ideally up high or down low, so she can't get out. To ensure your mum/uncle knows if she's up in the night, a motion sensor alarm for her bedroom door
For food - offer small meals, often. I stopped bothering about balanced meals - marmalade sandwiches were my mums diet staple, with cups of tea, for about 6 months.
When she's mean - diatract, don't challenge, just go along with whatever sceanario she's in, while assuring her she's safe. For my mum, I'd frequently shoo invisible cats out of the room, as telling her there wasn't a cat in the room was just too distressing for her.
I wish I'd got my mum a baby doll - I saw a couple of elderly women with them in a respite centre, and they seemed to calm them. I think I couldn't face up to such a clear visual symbol of how I'd lost my lovely mum, but I think it would have been good for her.
But most of all, please help your mum and uncle work out a way for her to move into a home. No one wants their kids to have to do this level of care.