couple of things
I find it hilarious that you started with a) then went to 2)
either way this is a purely cosmetic change because all we're doing is:
introducing a UI element instead of having it in the composer
stripping mentions from the plaintext but keeping them in the AP data (specifically, the tag field)
What we aren't doing is changing who gets tagged. Or well at least we shouldn't but it seems gabbo changed the logic probably as a result of rewriting some things (CC @gabboman cough we should tag everyone up the thread by default for parity with old behavior)
really what I think this was supposed to be is moving them out of the plaintext and into exclusively the tag field like what snac does
That said, that's a *key bug. If there is a mention for you in the tag field then *key should probably respect that 