Email sent from Thunderbird identity keeps 'from' field of the original account
Transfer (unregistered) OP's old solution to answer.
Gmail appears to ignore all aliases not set via Gmails own 'add another email address you own' function under 'accounts', so all outgoing traffic from Thunderbird under an identity is funneled through the original Gmail account.
Adding the address through Gmail (settings -->accounts and import --> add another email address you own then follow instructions) solved the problem: emails sent in Thunderbird under the identity appear to recipients as sent from community@gi--etc, and not [email protected]