If his brother tries to discipline you just say you've referred the matter to the college authorities and you can't say any more until they have acted.
I'm working I'm afraid. Monday mornings are busy because I have to go through lots of checks to make sure nothing went wrong over the weekend. I'm going to be busy for a couple of hours. But I can ping you when I'm free.
I'd be tempted to tell the brother that the college authorities have told you not to discuss the issue while it is being investigated. That makes it clear you aren't just refusing to talk - you have been told not to talk.
Possibly, but in any case it doesn't matter. Your best option is to avoid any contact with the idiot regardless of whether his elder brother is around or not.
As long as you're thinking about him you are the one with a problem. You can't stop him saying things about other people, or at least not without being kicked out of the college. So you just have to stop caring about him.