I honestly think that once a year is a little overkill, let it be a month sure, even three month cycles, but a full year is too much.
If the main issue of name changing is really impersonation with malicious intent (very important detail!) and breaking rules, then just warn/ban all the offenders, enforce said rules, so that people won't be so eager to break them. Report system exists for a reason.
Not sure about forbidden symbols, that are "intentionally designed to be hard to read", as the rule itself just sounds like too much work, because with such things you want to be extremely specific and draw black and white lines that are clear to everyone.
The 6 month grace period is complete reasonable, with one detail - if the owner of the old name changed his mind wants it back before 6 month.
To answer your question: When renaming an account is required due to rule breaking, should the account be locked until the account owner contacts the moderation team, or should the account be reverted to the last used acceptable username?
Yes, that would be the most sensible decision, just make sure that the communication to the moderation is accessible even for the most basic technologically advanced people and the locked account message displays a call to action for contact.