New Cat Causing Resident Cat to Scent Mark

Hi everyone! I recently got my cat Oswald (about 2 yrs, neutered male, declawed before I got him) on April 7th and he's very sweet. However, my roommate has two senior cats, Navi (14, F) and Link (11, M). Oswald is a confident cat and did fine with the introductions, but Link is very submissive and Navi is a reclusive and grumpy cat. Currently, Oswald's food and litter box is in my room and he's confined to my room frequently with 3-4 hours of exploring time per day.

Oswald has a bad habit that I've been trying to train out of him. He doesn't attack the older cats or anything like that, but he does try to play with them which often turns into one of the cats running away and him chasing them. This is incredibly stressful for Navi especially, who really doesn't like being bothered by anyone.

Navi has a history of scent marking and going outside of the litterbox. She did this when I moved in, and has done so when people have come to our house to laundry or anything like that. She's started peeing in the same spot on the stairs and has been doing so consistently every few days for the past few weeks. It's ruining the carpet and walls in my roommate's house (she owns it, I rent a room in it) and I just feel incredibly guilty and want to try to find a way to minimize this behavior. I try to watch Oswald as closely as I can when he explores, and am considering confining Navi to a room when he's outside.

I just don't know what else to do. My roommate uses animal specific cleaner and a green machine to clean the carpet, by Navi keeps going in the same place on the stairs. We've tried to use Feliway spray, and maybe just need to use more of it. Any insight would be greatly appreciated, I would sooner move out than rehome Osawld so I'm just trying to figure out my options. Thank you