Hello!

"I bet you weren't invited to this lavish do, I wonder how many went dressed up as you?"

"Joining your mission can come with a cost, this empty reminder shows just what she lost"

These riddles suggest Riddler at least knows who Bruce and Barbara is. What's odd is he acts surprised after Bruce becomes unmasked. Is there something I'm not seeing?

I don't know when exactly those comes up during the story, but, without context I can understand the first one as:

"...I wonder how many people went dressed extravagantly like a Bat-man"

for the second one, the fact that Barbara was helping Batman was known by many at that point, but wasn't necessarily clear how far she got.