i dont answer all your questions because i don’t know answers to all of them.
Metronome was (is, still?) a fork of Prosody. Both are XMPP servers. You can make users, mucs and all the XMPP things 
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I’m not sure why the switch has been made between Metronome & Prosody in YNH env. Prosody was ahead i think. I found this topic listing few differencies between the two
What XMPP do? well, a lot of things here’s a list of XEP (XMPP extension) supported by Prosody XMPP Extensions (XEPs) supported in Prosody – Prosody IM
If you were using Metronome, well it does the same but could do much more.