Am I missing something.. (spoilers)

Angier planned the performance as a trap for Borden. He would have heard Borden screaming trying to free his duplicate and the reaction of the public soon after using Tesla's machine. He only needed to wait a few seconds.

Besides, it was the last show, so a failure wasn't a main concern for Angier. His main goal was to frame Borden.

"You're not thinking 4th dimensionally!"

A) Borden's yelling was heard through all the silent auditory, and even on the balcony. Angier could hear them and decide that whatever happens there, not showing up would be safer.
B) Angier also could spot Borden when he was inspecting the machine with other invited viewers.

__________________________________
Oh my god! They killed Angier!
YOU BASTARDS!!!

Angier must have recognized Borden when the latter came up on stage to inspect the machinery. He knew that this was his chance. True, Borden might not have gone backstage, but Angier wss counting on his curiosity. If Borden hadnt gone backatage then Angier could have just emerged from his hiding place late and chalk it up to a botched performance.

He would if he is an exact duplicate with the same knowledge as the original. It makes sense that he would have all the same knowledge as the other Danton, thereby knowing what the plan is.