i don't have any advice to offer in regards to the behavior issue, but as for the cleaning, I have found a wonderful product that works extremely well. It's an enzyme based cleaner called Kids N Pets, endorsed by animal planet, and sold at (most) Wal-marts, or Amazon.com. I have a friend with an older kitty who likes to potty randomly around the house in his advanced years (I think it's protest at his long work hours, as he's been thoroughly checked for health problems) and even in high humidity, the treated areas don't smell. Wal-mart is your best bet money-wise, as the 32 oz bottle is about 5 bucks, U.S.
You do have to clean as much of the waste up as you can, then saturate the affected area and allow it to dry. The vents might be tricky, but perhaps some kind of fat sponge on a straightened wire hanger would be flexible enough (if you take the vent cover off, of course) to get down there to clean the worst of the pee off, then spray the kids n pets or whatever product you end up using liberally down and allow to dry.
hoping you can get things figured out and cleaned up! Good luck!