You can have multiple connections from the same IP, this helps even the load and you’ll be less likely to run into disconnects from maxing out the message buffer.
You can use the same Username/Auth token on multiple connections. This is how all large-scale bots work.
That’s because the message buffer is more of a common sense thing with the underlying technology rather than something Twitch specific. If your connection can’t keep up then Twitch aren’t going to just buffer and queue the messages forever for you, it disconnects you, the same as many chat networks if your connections isn’t capable.
As for rate limits, that’s per account. So if you were to create 10 connections using the Login/OAuth for ‘James’ as in your example, they’d share the same rate limits.