This casebook fiction of "BBW from a 2-point stance takes longer, allowing the ball to leave the FBZ" is not only questionable, it's also unproductive. The safety of a BBW immediately at the snap has virtually nothing to do with whether the blocker's hands were at his knees or on the ground - and even LESS to do with where the quarterback was standing.
Plus, it's difficult to officiate. Here's what goes on in my head each time:
Me: Whoa, a low block. Was that immediate enough?
Me: Yes
Me: Okay, but what stance was the guy in?
Me: Um... I forget
Immediate BBWs should either be blanket legal or blanket illegal. (Scrimmage kicks excepted.) That would be WAY simpler - and isn't Fed all about simplicity when possible?
How about simply changing the BBW requirement from "the ball has to be in the FBZ" to "the ball has to be in the FBZ OR the player must have initiated the block immediately at the snap"?