First one about the channels that have the main owner delete and there is admins in the channels
I suggest that the ownership of the channel goes to the one that oldest in the channel and active
Second thing is about the telegram usernames
There is many people that use glitch to hunt users and to scam people , I can explain everything but the best way to protect people usernames that they get a hint by telegram when they remove a username from their account or channels and they have a window about 30 minutes to get it back or switch it from a place to another like from a channel to another channel in their account
Other wise the username will get auto deleted that Noone can use it again or the username go to the fragment.com so it can be auction for it