Click on their profile. There is a “Send Message” button right at the top. You can see who the mods are by looking in the sidebar of the community.

Subscribe pending is just a visual glitch, you’re already subscribed but it just didn’t update visually. Posts from those communities are already showing up in your Subscribed feed.

Just start swearing at people. They will find you.

Depending on your client or UI, there should be a sidebar or something of the like that will list who the mods are, and you can message one or more of them from there. I’m not aware if there’s a way to send a single message that goes out to all mods of a community at once rather than messaging them individually.

As for #2, I think it has to do with how federation works. In my experience your subscription should still work even though it says it’s pending.