A: Yes, but only if you buy the "Green Hell + Spirits of Amazonia" bundle. The base game patch (2.9.1) updates the engine, but the DLC maps are locked behind a purchase.
A: Using a legitimate copy, yes (up to 4 players). Using a cracked Green Hell PC Free Download -v2.9.1- , no. Co-op requires Steamworks API integration. Green Hell PC Free Download -v2.9.1-