Hi folks! fyi, this is now resolved at the product level as the eng team has unlocked the ability to belong to multiple Cloud Retool orgs with a single email address, if you so choose! Could not invite 1 user with existing account in other organization errors should be no longer, and there isn't as much of a need to use the gmail alias workaround, though of course you can still use those aliases if you'd like 
See this update on the Feature Request post for more info.