I do feel for you, so this post is in no way meant to be upsetting-
From a buyer's perspective, I wouldn't touch your horse with a bargepole because:
-She has been out of work for two years, and although you say she would come back into work easily there is no proof of this, and taking her on for ridden work would always be a risk. As she hasn't schooled or jumped much in this time, I wouldn't take the risk that she would take to this again now.
-At 20, she may have 5+ useful years left in her, but she may start to show signs of aging very quickly and have to be retired or put down very soon. Age may also put off owners who have recently lost a horse and don't want to go through it all again too soon.
-Having been in the same environment for 5 years, I would worry about how she might adapt to a new yard at her age.
-She won't load, which really limits what you can do with her, and probably won't be resolved at her age. Even for hacking, many people will box to access better hacking, so loading is still desirable.
-Many people are wary of buying a horse they can't see ridden, especially if the owner has lost confidence. Not everyone in the horse world is honest.
There are lots of project horses available who are much younger than her, and therefore give the buyer more return for their investment (time or money).
If you are serious about trying to sell her, it might be worth paying someone to bring her back into ridden work. If it's easy, you may be able to find someone to do this for free, but if you pay, you will probably get a better quality rider. If people can see her ridden and coping with work, they may feel much more positive about taking her on. If she doesn't react well to coming back into ridden work, perhaps that might be the time to think about PTS.
I would say there is no shame in putting to sleep an older horse in the home she has known for 5 years, rather than her spending her later years being passed from pillar to post. The BHS do have a helpline you can call called friends at the end, with volunteers who give non-judgmental advice about your options- it might be worth talking to them?