That is GREAT that the kitty is using the box again. Not sure if he likes the bathroom as a location or if it's just that the bathroom is where he 'learned' to use the box. Maybe he thinks he supposed to only use that particular box and if the door is closed he just goes anywhere. If you want the bathroom to be a 'kitty free' zone you could train the cat to continue using that box but each day move it an inch closer to the door until it's eventually outside the door and then you can start moving it down the hall etc.. etc.. until you have it where you want it. Moving it from one place to another in one shot can be confusing, especially for a 'wee' guy. No pun intended. LOL

The same concept might work for you feral cat too. I'm glad to hear that she is back on the scene and hanging around the porch area. You could provide her with some treats or wet cat food and each night give it to her a little closer to your front door and then hopefully inside the door and back into the house at some point. She may also be looking for some time away from her kittens, especially now that she has been spayed. I know you said the other ferals are caretaking the babies but she may just want her freedom right now.