1) Regarding the kinds of mailbox names an email provider will provide you with, it's up to the provider. My employer, for example, only lets us have firstname.surname, With the addition of a single digit in case more than one employee has that name. These limitations have nothing to do with standards compliance.
2) Gmail does allow you to send mail to non-legit, but standards-compliant addresses like {^|~!}@example.com, because they know that their own mailbox name rules don't extend to other providers.
Regarding your point b) evidently they're not smart enough to grasp that addresses with + in them only work on the theoretical internet, and not the real one.
Completely different to standards-compliance, yes. However it is exactly the same thing as GMail's alphanumeric only rule.
To reiterate - the mailbox names that a email provider will provide are subject to the naming conventions defined by that provider. Whether the convention is firstname.lastname; 5-7 characters only; alphabetic only; or 13375p34k transcriptions of characters from Lord of the Rings. None of that has anything to do with whether they are standards-compliant.
The test is whether they will send email to any RFC-compliant email address regardless of whether it conforms to their own mailbox naming convention. Gmail does.
2) Gmail does allow you to send mail to non-legit, but standards-compliant addresses like {^|~!}@example.com, because they know that their own mailbox name rules don't extend to other providers.
Regarding your point b) evidently they're not smart enough to grasp that addresses with + in them only work on the theoretical internet, and not the real one.