Space in trunk by the fenders - trash

Thank you so much for letting me know! I've only been working on the passenger side, but it was completely full. Never thought to check for the plastic plug. That explains the rust there.
Started with a vacuum cleaner hose, but then just pounded the fender - got a small mountain of debris. A whisk broom moved a lot of the loose stuff around, but I can stick my finger in the hole and feel the dirt packed in there. Going to use the welding rod method and compressed air tomorrow.
I'm concerned about using water to rinse it, because I don't think I can get all the packed-in dirt out.

The trunk lid seal is falling apart. I have a new one, but have been thinking about having the car painted. Can I just push the new seal on without using adhesive, or will it keep coming off?