Use IMAP to copy/verify folder on another server

[rlebeau]

I assume some sort of magic happens which enables the transfer between IMAP accounts without actually sending emails.

IMAP is a mailbox management protocol, not an email delivery protocol (that is why you need a separate SMTP connection to send emails). IMAP has commands for adding emails to folders, copying/moving emails between folders, etc. Transferring an email between IMAP accounts is simply a matter of downloading the email from one account and then adding it as-is to a folder in the other account.

I discovered that there are quite a few emails missing, and trying to copy many in one go just doesn't work reliably.

Then you should contact Mozilla about that. Maybe it is a bug in Thunderbird, or maybe it is a bug in the target server, or maybe there are errors being reported in the IMAP exchange. Who knows. There is not enough information to go on.

So here's my proposal? Is it viable?

Sounds fine to me.

Getting TB to work with 365 involved an app-specific password; I assume something similar is needed.

Probably so, yes.