One Piece - Chapter 1164

Yup. Looks like things are going as planned. Teach seemed to know too much. “Before you were ready”. Looks like after his first failed attempt Xebec regrouped and realized that the Void Fruit was essential to beating Imu. Kuma saved toddler Teach+ his mom. Whitebeard adopted him years later (8-12ish by the looks of it, got to go back and see if his age can be dated). He adopted him from a ruined island (prob civil war or the WG hunting him). His mom likely died in that tragedy but not before telling him about his pops and the fruit being the key to defeating Imu, a fruit user who seems to solely depend on its DF.
Void fruit to nullify Imu, Quake fruit to shatter the Red Line. Teach is a the ends justify the means liberator. Luffy is a the means lead to the End liberator.
It’s very possible Teach tried to steal the fruit without killing the commander but felt he had no choice. As for WB, that was probably luck. A one time opportunity he couldn’t pass up in order to acquire the Quake Quake fruit.

as I see it either

A. He really didn’t know that Luffy was Ace’s brother so due to Ace rejecting BB’s offer, the following his capture and the chain reaction to the war, was unplanned but seized upon. It’s even possible BB did NOT want to kill WB but once it was clear WB was doomed, he yeeted in to steal the fruit. (Preventing it from likely ending up back in WG control as a bonus)

B. Blackbeard knew about Luffy because Ace brags about him (BB prob knows Ace’s lineage) So he specifically targets Luffy to draw him out to trade him for Warlord statues and draw out WB (who wouldn’t let the kid he thought would be the next pirate king die). After acquiring his new crew they head to the war and hide until WB is sufficiently weakened. Essentially the opportunistic coward we see is actually who he is. But a damn strategic genius.

When BB appears at the war it worth noticing how Wolf (the giant who walked them there) acts. He is almost mocking them for not noticing him sooner. So they def were waiting for something.

Either to see if WB would win, or lose.

While BB may seem like a wimp you must also remember. 1. His fruit doubles his feelings which includes pain. 2. Non-fruit BB is the only pirate we know to land a hit on Shanks. I think BB’s Haki is probably super strong, he is just pragmatic and hides it. He doesn’t take risky fights if he doesn’t need 2.

If your enemies/competition think you are a fool and weak. Let them. It works in your favor.