Does Defy counter Shadow Dash played from trash for flow cost?

Yes, Defy can counter Shadow Dash even when it's played from the trash via Flow.

Defy checks the printed cost of the target spell (2 Energy, 1 Power), not what you actually paid. Shadow Dash's printed cost is well within Defy's range (≤4 Energy, ≤1 Power). Paying Flow's alternate cost of 5 Energy and 2 Calm/Order Power doesn't change the printed cost, so Defy still has a legal target.

Per FAQ 11750 and 7464: cost modifications like Flow, discounts, or additional costs never affect whether Defy can target a spell — only the numbers printed on the card matter.