I can confirm these email APIs do not work in Forge, but not with any rationale. I hope support also explained that you can/should open a Suggestion (our term for “feature request”) in our open Jira (JAC) in the ECO project. I recommend specifying that Forge does not support endpoint in question. Once you have the issue key, please let us know here so other folks can watch, vote, and comment.
That’s another easier to confirm than explain. I have the first format. I think any new account will be created with the 2nd. All I can say is that we changed ID generators and, generally, Atlassian’s IDs should be considered opaque strings, similar to most of our security tokens.