Ambuscade, by its meaning, is basically an ambush. If two characters, Alice and Bob, are interacting peacefully, and Alice suddenly attacks Bob, Bob does not count as surprised (as I understand it. Correct me if I'm wrong). However, what if Alice is a Ranger (UA Alternative Ranger) with Ambuscade? Would this mean that Alice is suddenly "ambushing" Bob out of nowhere? Or would Bob still count as unsurprised because he was aware of Alice's presence?
Now, if Andy was being stealthy and went unnoticed by Ben, and suddenly attacks Ben, Ben is definitely Surprised. If Andy is a ranger with Ambuscade, when does the surprise take place? Does Ben count as surprised during Andy's special turn with Ambuscade, or does he count as surprised in the normal turn after Ambuscade, or both?
The reason I am asking this is for an Assassin/Ranger multi-classed character. As Assassin, the character needs to have the enemy surprised for an automatic critical hit upon hit. I am wondering how the character's Ambuscade feature (as a Ranger) affects this.
I am asking this from the perspective of a player, not a DM. I am not looking for answers that start with "As DM, I would...". I am looking for answers that are based on real sources (including UA) and written rules.
I know that nothing about the Ambuscade feature itself says it affects surprise (of the enemy). I am looking for citations of rules about Surprise itself that would give clues as to whether or not Ambuscade would logically affect it. For example, a feature that guarantees a Stealth check success would not state that it affects surprise, yet it does.