Hi, I'm mobile and have done lots of old/disabled clients hair. I brought a portable backwash that is height adjustable, so you can really push it into their neck as far as it will go.
https://www.ebay.co.uk/itm/133554078990
(Mine is the large size)
I have a bucket that the hose goes into and another bucket that I operate my camping shower from. Absolute game changer! It's definitely my unique selling point, older customers love as they literally just sit down and I move around them, and as far as I can tell it's pretty rare for mobiles in this area.
I have one lady whose head tilts consistently to the left so no matter how hard I push in she's watertight on the left but big gap at the right. So we use 2 plastic capes, and tightly roll the towel around her neck at the top to try to fill the gap a bit (obviously need to change this after.) When I'm rinsing I cup my hands around the gap area so the majority of the water runs away to the back. Keep asking them to push/lean back if you have to, I usually have to say it at least twice per wash! They don't mind.
When I was at my salon job we mostly did OAPs who often couldn't get close enough into the basin, so you got very good at waterproofing! My extreme cases would be gowned up like this -
First, a tight fitting water proof gown that the back went over the back of the seat. That way if water did get through it would just run down onto the floor, not down the back of their chair.
Secondly, a disposable plastic cape (the short black colouring ones)
Thirdly, a flat towel around the neck.
Fourth, another towel but tightly rolled a few times at the neck area, all secured with a clip.
The ladies who it happened to were usually used to the varying results of wetness from which ever junior/assistant washed their hair, they were mostly fine with it and I often had a laugh with them about it, (eg "right now to waterproof you up," and "yay! We didn't get you wet!") A little bit of humour definitely helps, if it's appropriate of course.
All the best