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.
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.