i can defy public execution. but if they flow cost it, can i defy it still ssince its flow is 5?

Yes — you can still Defy Public Execution when it's played from the trash for its Flow cost.

Defy only checks the spell's printed cost (2 energy, 1 power for Public Execution), never what was actually paid. The Flow cost of [5][A][A] replacing the base cost is irrelevant for Defy (Rule 206; same logic as Repeat and cost reductions). Playing it via Flow doesn't change that it sits on the Chain as a normal spell, so it's a legal Defy target (Rule 829.1.b.2).

One wrinkle: if your Defy counters it, Public Execution gets banished, not returned to the trash — Flow's "then banish it" is a delayed replacement that fires whenever the spell leaves the Chain without its own execution doing so (Rule 829.1.b.1).