Finally, everything is working now. The final SMTP problem was one that I unwittingly created,...
When I deleted and recreated my AquaMail app-specific password during troubleshooting (because I have 2FA enabled), I updated the password for the parent Zoho account but -- being so fixated on incoming IMAP traffic -- forgot to also update for my child identities' SMTP credentials.
(._.)Well, at least our international nightmare is over.
Thank you to all [!]
EDIT: At least I'm now 28 messages closer to the next badge after Newbie. LoL