I generally agree with this assessment. I think Britain would have a hard time abolishing slavery in the southern colonies, however. While it is all very speculative, I can imagine the southern colonies rebelling in a similar manner as they did with Lincoln & the northern states in the US Civil War. Britain would probably win that, but I imagine it would still be very bloody.

Also:

The US would probably operate a parliamentary system like the other dominions, which I think is a better system that is less open to executive abuses

I have thought this on numerous occasions. While a Trump-like figure is still possible, they would be less of a threat to the overall system.