To answer your question about boots 24/7, the answer is no. Boots can be worn for at most 8 hours a day. Beyond that, you're just asking for rubs, thrush and other nasty things to get going inside the boot.
I can appreciate your frustration. There's only so many options, and I agree what's the point to keep a horse you can't ride. If you aren't emotionally attached to her, sell her back to previous owners.
Do you have any pics you can post? Perhaps there is something obvious about her current trim that might suggest a way to improve her feet.
Correct that you cannot leave boots on 24/7 but I am going to stretch the "at most 8 hours a day", based on my own current experience. Before I tell my story, if you can't get to your horse every single day and clean its hooves, what I do will absolutely NOT work:
One of my horses foundered pretty bad in early March. He goes out to pasture every day in boots with home made Lily pads. He wears his boots 8 - 11 hours every day.
He comes in at night and has mats in his stall with clean shavings on top of the mats.
Every morning I pick his hooves, brush them out with a stiff bristled brush and pour Absorbine Hooflex "Thrush Remedy into the white line (he has wall separation from the founder but no WLD) and I pour it into the collateral grooves.
At night I bring his boots to the house and wash the insides with hot water and Dawn dish soap. I generally get 4 - 5 days of wear from the Lily pads I make and I also wash them in hot water/Dawn dish soap every night.
To repeat, this epic and excruciating labor has been going on daily since mid-March and no let up in sight due to some other issues, thanks to a very expensive farrier that did not do his job right.
If you are willing and able to devout every day, every day, every day, to the care and keeping of those hooves and keeping the boots clean, you can turn the horse out daily for up to 10 - 12 hours.
24/7 turnout in boots is a big No-No.
If you can't do that, for whatever reason, then ditch the boot idea because MyBoyPuck is right in that you are asking for big trouble in a whole bunch of ways - starting with thrush
Ditto, it might be best to sell the horse back to the previous owners, since they are willing to take it.
In my case, I am retired and, due to some nasty injuries, I am not able to ride anymore.
This horse is 17 and the baby of my four. As long as he is not in pain, I will continue to doctor him and, he will be a pasture ornament until he says he's ready to meet his ancestors:)