Thanks very much for that thorough summary of related RFCs that I hadn't reviewed recently. It was quite helpful and informative.
My main take-away is that my guess based on observations was pretty accurate. I think I'll choose to be encouraged by the statement that domains for e-mail addresses are restricted to mundane characters and to take it as reason to ignore (for now) the hints that arbitrary binary labels might need to be supported.
As for the comments about my decisions being "internal", they are actually decisions about what forms of e-mail addresses we will accept from any user on the internet who wishes to register for some of our services. So it isn't strictly "internal". But I also think that it isn't something strictly covered by these RFCs (we aren't using the system to implement something that talks SMTP, for example). The protocol involved is "accepting text from random users over the internet", so I think some simplification is warranted.