How can I get iPhone Mail to accept meeting invitations attached as .ics files?
When I receive an invitation to my Gmail account, it auto populates in my Google Calendar with a question mark beside it.

Unfortunately you can't accept - however it will now show in your calendar.

If the invite is sent from gmail, there is a link to select if you want to accept the invite.

Not exactly the full answer you're looking for but, hopefully this helps.