The dude was legitimately clueless about Ace being here for a vengeance, to avenge Thatch, when he confronted him on Banaro Island, until Ace attacked him.
Not true at all. Bb knew exactly why Ace was there. Ace said that Teach knew exactly why he was there (to kill him) and Teach acknowledged it. (In viz teach simply says “yeah…”)
“ Ace: Let's skip the pointless act, Teach. You've been around TWICE as long; you know what my being here means.
Blackbeard: All right. Fine... Let me just say one thing, then. Ace!! Will you join my crew?!!”
Augur was also the first to attack. Not Ace. Everyone present knew exactly what was about to go down.
And that's not counting the fact that he recruited the most twisted and untrustworthy criminals of Impel Down into his crew, based on strength alone which is basically an invitation for them to betray or abandon him should things go badly for him.
Teach lives dangerously but he’s aware of the risk. Even after the timeskip, he didn’t trust shiryuu. For his goals, he needed to quickly assemble the strongest, most dangerous crew possible. Impel down was the best avenue.
Kuzan is likely another marriage of convenience. Both of them are likely planning to backstab the other, both expect it.