missing mailboxes how to recover?

had been running Yosemite for a number of years, and have been using Apple Mail for forever

(2 Gmail accounts, 1 iCloud account, archives ON MY MAC mailboxes)

recently thought I seemed to have lost a single mailbox from one of the Gmail accounts, sidebar 😕

Seemed odd, scratched my head for days, convinced I def HAD that mailbox once upon a time and so went digging in my Time Machine backups.

<27 Sept I had 17mailboxes in that Gmail account

>5 Oct I only had 10!!!!! 😮

No idea what occurred during that period, no major crashes or fails I recall, but thats when they appear to have vanished.

Unfortunately I didn't notice until after 17 Nov when I upgraded to macOS Sierra (not high).

The FULL backup I took of Yosemite on that day does not include the missing 7 mailboxes if I look in ~/Library/Mail/V2/iMAP-account

I've tried all manor of methods to get this mail back but i'm at an impasse it would seem.

I've tried pulling back JUST the Mailboxes but they wont IMPORT,

I've tried resetting Sierra mail and setting up from scratch with the recovered V2 directory (just gives me the latest version of my mail with seven missing)

Resorted to restoring a FULL time machine to the 27Sept and the 20 Sept, on a seperate drive.

When I open Mail, it displays ALL seventeen mailboxes but NONE show content.

...the moment I put the network interfaces online the 7 boxes vanish! 😟

I can open the app up with the mailboxes visible, but the network has to be off.

ANY tips on how I might get into the contents of these mailboxes?

they all show various sizes in Get Info, so it would appear there is SOMETHING lurking in the mbox files

frustrated dot com