Basically, MJ is a special needs cat and she has very little chance of getting and keeping another home, plus it would traumatize her. GoGo probably could get and keep another home -- and maybe you might consider this because it is the only solution that has a good chance of success. Too bad it's been going on for so long. MJ might have turned against all litterboxes forever because of it. The fact that she's peeing and pooping everywhere, not a good sign.

You're going to have a baby soon and that will be an additional stress on you and the whole family and you just won't have the time and energy to monitor all the cats' interactions when you're home. I don't think having GoGo outside half the time will really fix this. I'd really suggest that you get GoGo spayed and place her elsewhere, and then create a personal litterbox for MJ that's very different from any box she's used before, put it in a different place away from any others, and fill it with something different too. And introduce her to that gently and persistently. Her feet might be sensitive, can you tell? Even though it's an extra expense to get special fill, you won't have to do it forever. Those pine granules are great and extremely good at controlling odor -- I use those when for some reason a cat has to have a litter box in the house (otherwise they're in the garage), and that way I don't smell it. It lasts a long time too. I don't use a full-size box, I use a small dishpan-sized box. Plain sand is also good. Experiment and see what she likes, and I know she'll appreciate the extra attention. Then you gradually mix in your regular litter (unless you discover in this process that the litter hurts her feet, which might be happening on top of everything else). This is a life that you saved, and I know you feel a special responsibility for her as a result. If you act quickly you can hopefully get things back to normal before the baby comes. After that you can forget anything but baby for awhile, lol!