The reason you can’t currently follow that account is because it’s a bot. We haven’t added follow support for bots yet.
This seems to be due to Mastodon’s Authorized fetch being enabld on mastodonapp.uk. I’ll look at that again in the next day or so.
I’d note that when you follow a hastag on Mastodon it’s only going to show you posts with that hastag that are already on the server you’re on. If there are posts with that hastag on another Mastodon server you’re not going to see them. To put it another way, Mastodon has not added primary ActivityPub support to their hastags (e.g. they haven’t made them Actors like the Discourse Plugin does for Discourse tags). See the docs and the discussion here. It may still be possible to do, but it will require some kind of relaying or perhaps polling.
You’ll need to share the message itself for me to tell you more (i.e. not just the backtrace). When you have verbose logging enabled the plugin will log failures to process incoming objects. There can be many reasons for a failure to process, many of which are not related to Discourse. The fediverse has a lot of objects flying around all of the time. I may change the log level of these failures to process to just “info” as I think they suggest to people a problem with the plugin itself when in most cases there isn’t a problem on the Discourse end of things.
Yes, they should appear as replies on Mastodon. Specific examples are always best as I can analyse what has happened and can address anything that needs to be addressed (if anything does need to be addressed).
Yes, that is how the “Full Topic” setting works. See further