Q: Can I use this if I work at a Microsoft Teams enterprise org? Probably not, I'm guessing, but hoping there's a way to make that work somehow.
Do you mean Microsoft 365 for Enterprise? Teams is the chat app.
Thunderbird can sync with Microsoft 365 for Enterprise mail servers and can be configured to store email in a maildir format. So you could use Himalaya to read emails that Thunderbird synced, but you can't manipulate Thunderbirds maildir.
I haven't tried, but the README mentions Office 365 and OAuth, so perhaps?
Depends, likely not as Microsoft strongly recommends disabling IMAPv4 and POP3 access to mailboxes in 365 and I think it's now the default. Most company policies do not allow it to be enabled outside service mailboxes.