That said, solution #2 (multi-comms) is considerably better than #3 (comms following comms).

the problems the author associates with #2 are easy to solve, if users are allowed to share their multi-comms with each other as links

Additionally, multi-comms address the root issue. The root issue is not that you got duplicate communities; it’s that communities in general, even without duplicates, are hard to discover.

I respectfully disagree. In two minutes, I can easily find all the communities on a given topic and subscribe to them all. The problem is not discovery. The problem is fragmentation of the user base, as explained by popcar in their blog post:

Alright, time to post. But where? pancakes@a.com and pancakes@c.com are both somewhat active... Should I post in a and crosspost to c? Maybe there's hope in other communities kicking off again, should I crosspost to b and d as well? Oh no, am I going to post 4 times just to find my fellow pancake lovers?!

Let me take this a bit further: After crossposting to all 4 pancake communities, I get three comments. One in a, b, and d. Each comment is in a separate post and none of them interact with each other unless the poster opens each crosspost separately.

I do not see how Proposal 2 (multi-communities) solves the issue of fragmentation of the user base, while Proposal 3 (communities following each other) solves this quite elegantly.