Bathroom sink odor and wet walls!

I wish I had some pictures, but I don't want to crawl back under the sink until I go to Lowes again! Here's the deal:

About 20 years ago, the previous owner of this 1952 home added a bathroom at the end of the hallway. Along with some other interesting choices, the renovation did not include any kind of accessible venting, and there is no obvious vent on the roof. I don't think there is an Oatey in the wall, but there is no way to access it even if there was. The bathroom walls are tiled, so no easy access to anything from that side.

Under the sink, the plumbing goes to a wall that is shared by a bedroom closet on the other side. There is an access panel under the sink, with a cutout for the drain pipe, and when I take it off, I can only see the PVC pipe go to a rubber flex joint and then pass through a vertical stud. No visible vent or anything.

After clearing a sink clog with a plunger a month or two ago, we started getting a swamp gas smell from the sink, and the toilet flushing would gurgle the bathroom drain. I got an Oatey vent kit and replaced the PVC pipes under the sink, but the odor is still there. It's actually coming from the access panel opening under the sink, from inside the wall.

I went into the neighboring closet, thinking that I would need to put an access panel in there to investigate the source of the smell. Well, the plaster wall is wet and discolored, all along where the drain pipe is between the walls. I'm *definitely* going to need to cut out part of that wall and see how bad it is.

Questions for people who know more than me:

1. Primary concern is water / sewage in my wall. What kind of mold / mildew control should I do once I have things opened up?

2. The actual plumbing repair should be the easy part. I bet I popped off one of the flex joints in there when I plunged it, and it's been leaking. If I find an AAV in the wall, would I be ok capping it off, since I have a new one under the sink?

3. I am not, and I repeat, *not* a plumber, just handy enough to install dishwashers and disposals. If you've got thoughts or advice, I'm open to it.

Thanks!