Re: mod concerns. A user can only change their name once in a 60 day period. The abandoned name is locked for 6 months.
Re: IDs. These never change during a name change. We made v5 explicitly to support name changes, which is why we moved to reference by ID in the API and put out the action required communications.
Re: historical data. This is something that’s been discussed. Nothing to share right now, but we’re aware of your concerns. I’ll keep you posted as we come to decisions about it.
Re: chatters endpoint and using IDs. This was covered quickly after the v5 announcement. It is question #3 in the FAQ: Twitch API Version Update Discussion & FAQ
Re: initial subs. Since it is a message from an “account”, this will be a little tricky. We’re working on subs and how they’re represented but nothing to announce immediately. 
Re: name change notifications for bots. @xangold could speak to that better than I can. I’m chatting with him about the JOIN/PART/ROOMSTATE thing offline. We’ll come back to you once we figure out what makes sense, if anything.
I’m going to be at GDC all this coming week, so I’ll get back to any further questions as soon as I can! 