The ones I left out from your original List are a “Yes, DUH!” ;D
Considering I want this whole thing to be able to run from a USB Stick and Raspberry Pis (never save tons of Data to the SD Card, it will wear out), yes very much.
I would say “just make symlinks to where you want it to be”, but Windows is a thing sooooo…
Very unsure about that one. Sure the Model Editor definitely is planned to be used there, but a whole Dev Toolkit I am not sure about. installing a new Dev Project however so you can use it with IDEA or so is definitely planned though.
Something like that would indeed be nice if it was possible ingame.
Unsure about that one, I would need help integrating something like this.
I plan for that to be doable Ingame, otherwise you should just use a Browser. 
I would like to stay independent of Account Systems, I prefer Cryptographic Authentication with SSL, so your Identity is bound to the Cryptographic Keys you generate yourself. That way you can just tell a Server Owner “Hey my Public Key is XXXXXXXXXX can you whitelist me?”, and you then can join just fine using your Private Key.
I went more in-depth on that other Post of yours, but yes that, and you can add more Databases yourself should you desire to.
Matrix for Chat/Voice Clients, and I would rather not touch Discord or Amazon at all.