My 25yo QH has bad arthritis in one hock. I use the magnetic hock boots every time I ride- I put them on as soon as I bring him in to groom and such. I'd always heard you weren't supposed to leave them on more than a few hours and not overnight, but I guess if people do it then it can't be that bad? I've left them on like 4 hours before and his hocks were warm when I took them off.
I also give Pentosan injections myself which are a great help! The oral supplement that I have personally found to work the best for me is Flex Force. It has HA, glucosamine, and MSM all in it.
Arthritis is definitely not a "death sentence," it just requires a little more care and management but it can be dealt with. At least you know for sure now and not later, whereas I found out many many years into it where it had gotten to a point of bad bone spavin where there was no option to get into the joint for hock injections. -.-