You (should) be able to play it as a demo in steam Next fest right now, but Steam has been a complete clusterfuck for the last week so it may or may not actually appear. If you search for it in Next fest proper it may show up. I'm waiting for that to be fixed before I announce it.
No, Metatron and Maya have gotten a good amount of content already this update and they all will get more. Sam has an eventual(tm) CG upcoming as well, and Ashley's is in progress right now. Many of these new characters aren't meant to be as fully fleshed out as the party members, but expand the setting (and have some lewds - Rosalyn's CG is in progress).
The party members are still the main draw, don't worry~ Regardless of if you prefer Mega or Meta heh.