Can't help with any of your questions apart from saying that I think the hydro pool is a great idea, your dog would certainly get the exercise, but what of the cost??? Can't imagine what it would be to pick up and collect and use the pool

but I bet it wouldn't be cheap, Also would they warrant a dog just going there for fun?? (forever pesamistic) I'm sure if you ring around you may find a dog walker who is very apt to dealing with dog aggression, hopefully someone in your area will come along soon.
But to be honest, if your mum and dad are going to do morning and evening walks that may well be sufficient anyway, your mum is only away for 3 days a week, so could the dog walker for your other dog, not just let your Viz out in the garden and stay in the home for a while.