As for cleaning the skin, you can use wipes, or you can do a butt bath. You may need to do both, depending on the size of the mess.
Paralyzed pets do not usually have blowouts, wonder if it was from medication.
Keeping him in the bathroom isn't a bad idea, might be easiest.
It sounds like he got some injury in the lumbar or tail area, I'd want to be careful with his tail when you put on the diaper since it's healing.
Sounds like the vet said you should expect improvement in 2 weeks? A nerve injury can continue to improve for much longer, months and even years, it's gradual.
Here are some videos about diapers.
This video shows taping the tail hole to keep the filling in. It may not be necessary. If the absorbent filling of the diaper is like grains of sand falling out, I'd tape it. Some diapers have a different filling that stays in the diaper pretty well even if it is cut. You'll know when you cut the tail hole. If you have little beads of sand on your table, it probably needs tape, otherwise I wouldn't bother.
This one, she does not tape the hole.
This one shows squeezing the bladder to make the cat pee, then stimulating his bottom to make him poo (cat is paralyzed, not just injured). It explains why she doesn't want him to poo in the diaper. She puts it on with the tapes toward the back, which works better with a lot of animals.
Here is a video about a cat that remained paralyzed and is still enjoying life.