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Does an option trading below parity always indicate an arbitrage opportunity?
Probably but not necessarily.  Your question could also be posed regarding cash & carry for commodities in contango: If I can take delivery on the gold now, short the gold next year and make delivery then, paying the storage fees, is this an arbitrage opportunity? It is in the sense that you know your delivery and the money you will make, but it's not in the sense that until delivery (or execution in the options case) you are still on the hook for the margins due from price fluctuations. Additionally you need to consider what ROI you will make from the trade. Even though it's "guaranteed" it may be less than what you can earn from other "zero risk" opportunities.