question

I need to build an incremental local backup/archive of all messages transiting (incoming/outgoing) a webmail account.

requirements

Most requirements derive from the fact that I need to build a local backup/archive (one-way sync only!), not a mirror (two-way synchronization).

All messages in all folders must be included in the backup: Inbox, Sent, custom folders, etc.
Remove deletions must not cause local deletions (otherwise it would be a mirror, not an archive/backup).
Never create local duplicates (i.e. all messages must be downloaded and stored locally exactly once).
Messages must be stored as full offline copies locally (not just headers or pointers).
I must not interfere with usage of the webmail account. I.e. the account must be left untouched and no trace must be left there.
Read‐only access to the server: no moves, deletes or flags remotely. Local move/delete operations must not be reflected back to the remote storage.
I would prefer to use the POP3 protocol (because it is more straightforward, fewer Thunderbird bugs and because it is designed not to mirror local operations back to the server). But to my understanding, POP3 only downloads messages from the Inbox. Is there any way to back up the remaining folders also with POP3? If not, I will probably have to use IMAP.
I absolutely want to achieve this using Thunderbird, if possible in Thunderbird out-of-the-box, otherwise using addons and companion programs like Total Commander. If it cannot be achieved in Thunderbird at all, please explain in detail why and suggest other methods or freeware Windows tools.