Every time Peggy pisses on the floor I break down shortly afterwards and sob for an hour. It's distilling all my grief for Charlie.
Sucks.
I'm sorry to hear this is happening frequently - is it possible that this is why she kept ending up back at the shelter?
My Jakey, bless his little cotton socks, has an extremely loose bladder around fresh laundry and bed linen. You can pretty much see the glint in his eyes as he spots a pile of laundry. It can be really exasperating at times. He is the reason I now have a large capacity washing machine, so it can take our heaviest duvets. I love him dearly, but it is annoying (although I am kind of used to it now).
You need to scrub any areas of wall/carpet/skirtingboard where she has pissed with a solution of biological washing powder in warm (not hot) water (wear a pair of marigolds in case it irritates your skin). Bio detergent breaks down the proteins in urine so it can no longer be detected by cats. Better, if possible, to get a product called "urine-off" which is a spray on enzyme cleaner. Spray it on, leave it for a bit, and wash it off. Once cleaned, spray the area with Feliway.
If you catch her in the act, say "NO" firmly but without raising your voice, and calmly put her in a litter tray, then give praise for being in the appropriate place.
Have you tried a different type of litter? It may be an idea to go back to basics and get some sand (not the highly orange stuff that builders use, but the non-staining stuff used in kiddies sandpits). If she wants to go in corners, put a litter tray there.
EDIT: Oh and trust me, I love Jakey to the end of the world and back, but there are times when I have been in tears because I went to go to bed only to find that I was lying down in a puddle of piss and had to do a shit load of laundry before I could sleep. It can be really frustrating.
In behavioural terms, I would say it is due to territorial insecurity (which is often more of a 'thing' for female cats than males). I have had mixed results with Feliway, but it does help in some cases.