Um, I don't think it was incompetence, there were quite a few polls showing gammons would be happy to see NI (and Scotland) go if it meant they could secure Brexit - IMO it was a calculated gamble that paid off handsomely in the 2019 GE.
You're right about him shafting the DUP about a fortnight after saying there would be "No Surrender" at that years Tory Conference, how the DUP laughed at the time, as the saying goes, they're not laughing now.
I think BJ is quite happy in his role as the Lord of Misrule, safe in the knowledge he'll be gone when the brown stuff hits the fan...