I'm so sorry you are going thru this and glad you found Cat Attract!!
I have a sick cat that is going thru the same problem right now and picked up some Cat Attract additive the other day. It came with a brochure that brought up a lot of the points already discussed in this thread and I'll try to breeze thru the highlights. Sorry if you already have the brochure and read it, just posting it for others on the sight also.
1. Use Cat Attract litter
2. Buy new pans and clean litter
3. Put the box in a different area that is in a quiet area, away from bright lights, loud noises, vibrations from washing machines or furnaces and away from an area that could be disturbed by kids and dogs.
4. Destroy old smells
5. Put food where they are going
6. Put the cat's smell there (rub them with a towel then rub it on the area)
7. Use Feliway
8. Get rid of hooded boxes
9. Reduce stress: play with them for at least 15 minutes each day, put their condos in the window, play music/TV for them while away, hide treats for them to hunt.
10. Rule out medical issues
For the really tough case (sounds like your girl):
Put the litter box with Cat Attract in the problem space for 10-14 days. After this time, move the box an inch or two each day toward the place where you would like it to be. Alternatively, confine your cat for at least one month to a room that hasn't been soiled in the past. Provide a litter box along with food, water, scratching post, toys and elevated hideaway. After a month the cat should be consistently using its litter box and you can expand the territory to include 2 rooms, then 3, and so on. Over time most cats respond to this treatment. Some require a more rigorous treatment: Confine the cat to a large cage or pet carrier that is elevated off the floor on a table or bench. Cover the entire floor of the carrier with litter. Create a small bed and spot for water and food at one end. This will force your cat to use the litter. Over time, reduce the amount of litter in the carrier, so that the litter only covers a portion of the floor. Once the cat is consistently using only the litter covered area in the carrier, bring it out into the room and proceed with the one-room confinement treatment outlined previously.