I've been following this thread from day 1 - having the same issue on both my iPhone 6 Plus and iPad Air 2. I've tried everything recommended - nothing works (as we know). I've had 6 chat sessions with support - their response has ranged from delete the email account to Apple engineers are working hard on it, to this is the first we've heard of it. So no real support with that.
It's been escalated twice now, and I've got a phone appt with Apple on Wednesday - see what comes of that.
For me this is a critical issue - when on field trips away from phone reception areas (i.e 300km away in the bush - with no phone reception) I need access to email (& attachment) content.
Yesterday I attempted the restore as a new iPad fix (so called) - the emails worked fine for around 20 hours, but back to the problem again now. Email storage went from 100mb down to 2mb.
Hate to say it, but if not fixed may have to go to a Samsung - I have had every iPhone from the very first (and every iPad), but this is impinging on my ability to work away on field trips big time now. Deal breaker