What I've noticed:

• The posts themselves follow almost identical structure. Large quotes with poster emphasis.
• The posting topics is almost identical.
• The reliability of sources is hit-or-miss on all posts. Some are from legit sources, some are from really questionable ones. The questionable ones are common between accounts.
• The communities where they're all active are the same. Lately there's a bit more separation where some accounts frequent some communities more than others. E.g. some time ago we used to get Hotznplotzn, randomname and Scotty in !Canada. Now we mostly get Scotty.
• When you engage in conversation the lang expression, attitude and arguments are identical. This when I really noticed the pattern.
• I've had multiple accounts from this set group up/down vote their/mine comments, deep into a discussion that didn't attract other up/down votes.
• I've had a discussion that reached a dead end with one account, only for another to show up and restart it from a different angle attempting to reach a different conclusion. E.g. first discuss an economic side of some China-related issue, reach a dead end, restart with human rights abuses side on the same topic. That's while having the group up/down voting action going on.
• Two of the accounts were created on the same date, on two different instances, a few minutes away from each other. This was the smoking gun for me that this is the same person.

Some of these aren't damning on their own, but put altogether make me believe it's one person. Also they never deny that when pressed. The conversation just stops and they disappear for a day or two until the next post.

a disgruntled Hong Kong exile with too much time on their hands

Quite possibly. I think they may live in Germany or be German because I've seen some activity in German. Who knows. I doubt they're a paid actor because there's enough money in the official media machine pushing this line so I think you're right. Someone who really hates China/CCP/CPC, perhaps for a good reason of their own, with a lot of free time. It really sucks because there are really interesting discussions that can be had on any of these topics. There's another guy around here whose family emigrated from China because they weren't having a great time with the 1-child policy among other things. He's in the US and can have rational and interesting discussion about this stuff without bursting in flames.

E: Here's a recent unhinged discussion with Scotty.