Sucks, right, because on the theadiverse, you're not actually able to do that so easily.
Sounds like an unnecessary limitation of threadiverse software. Why limit a post to only one community? That doesn't make any sense.
The person who made the post with multiple mentions clearly did it intentionally, and I would do the same because for every topic I am interested in there are 4-5 groups on different servers.
Every mentioned person gets addressed
In most cases, this is what a user wants. Some platforms support silent mentions, though (Friendica, if I remember correctly).
hashtag / community tag soup
I think this should be viewed as a moderation problem, not a protocol problem. If you don't want to see mention soup, just limit the number of mentions per post on your instance.