I realised from your earlier post that you have way more experience than me and I'd wandered into 'granny/egg-sucking' territory. It sounds like you've really got your work cut out this time. My Mum was always a tough character to deal with, but there was no history of physical aggression which had 'trained' her to stick up for herself when backed into a corner. Plus I'm a bloke, and testosterone gives me an advantage when it comes to controlling a situation... which makes me sound like a right so and so, but I think you know what mean. Sometimes just being a bit taller and stronger allowed Mum to take her frustrations out on me without me coming to any harm.
Well, not much harm anyway. Scars fade.
Your son's gran's dementia has backed her into a corner and although fear and confusion's what's keeping her there, it seems unlikely simply being nice and patient is going to change her. I have lorazepam for emergencies, and that -- in small enough doses -- definitely reduces anxiety and can sometimes allow me to get Mum's dentures in and out without too much of a fight for essential cleaning. or simply to calm a turbulent day, allowing trust to be rebuilt.
Showering and drugs don't mix though. My mother was a hairdresser and even getting her to wash her hair is a nightmare most of the time. I gave up with regular showers or baths a long time ago and bought her some Chanel No. 5 instead! In a toilet emergency I'll insist on a shower, but it's no fun for either of us, and I hate reinforcing her fears by 'bullying' her through a quick all-over wash. Seems counterproductive.
I'm lucky though now, because me being tolerated in the loo, and when changing clothes, means there are opportunities to warm up a few wet wipes on the radiator and gives legs and feet, or armpits -- or anywhere else that soap hasn't visited for a while -- a quick wipe. That's not going to work for you, and I think until you've got some chemical help from her doc it's unlikely to change.
I've mentioned in another post that I'm currently trialling CBD oil on my Mum. The high strength stuff (necessary because hiding two drops in some fudge is easy, hiding 15 drops of weaker stuff is much harder!) is silly-expensive but I'm increasingly convinced that it works... at least for my Mum's anxiety and aggression. There are no guarantees with anything, and it's possible Mum is a bit less stable on her feet with the CBD oil as well.... I may just be paranoid though. I feel guilty about 'experimenting' on her, but... grief, her life was no picnic before, with one source of disorientation and confusion after another. She may be a bit sleepier now, but she also smiles a lot more and is more malleable for essential tasks.
Well, some of the time. Nothing's perfect. But when I stopped the CBD oil for a couple of weeks, the smiling mostly went away too. Anyway, I'm very uncomfortable with this sort of recommendation, but when folk are struggling with aggression I think it's fair to try just about anything that might help. Of course if anger levels are high, the marginal effects of CBD oil may get easily swamped by fear and adrenaline, and you just end up pouring money down the drain. But if your son's gran still has capacity and medical intervention isn't a simple option, sneaking some CBD oil into something may be better than nothing.
The oil I'm using now is 4000mg in 10ml, which is as high a concentration as it gets and way higher than the 'cheap' stuff in Holland & Barrett etc. This means it tastes more strongly, which is a bad thing, but a drop or two hidden inside some fudge (likely to stay in the mouth long enough for some of it to be absorbed 'under the tongue', which is the recommended delivery method) is easy to deliver. Twice a day seems optimal for Mum, but her aggression levels are nowhere near as high as they were. These days it's just muddly reluctance... no stage lasts forever.
Maybe that's the only thing you can really hang onto. Your son's gran won't stay like this forever. You just have to keep your sanity for long enough for that to happen. Easier said than done!
Anyway, I'll quit wasting your time with my well-intentioned waffle and say I think it's admirable what you're doing for someone who's not a direct relative. However part of me wonders if you've done your bit already in life, helping so many in the past. Maybe it's time you started looking after yourself, even if that means allowing your son's gran to 'fall apart' so social services are forced to take a greater part of the responsibility.
There are never any easy answers, are there? Good luck.