Yes, Ironmen can. At some point this was patched to allow ironmen to trade the requisite items between each other so they could actually complete the quests.
I don't think you can help after completing the quest if you can't do it normally. Your best bet would be to try an Ironman FC, I guess, and maybe they could partner with you or help find someone? Or world 3? Not sure.
Not necessary between ironmen, an ironman player can do the quest with a non-ironman player as well.
Also, at the TS: I am not fully certain but I thought people can help more often with the Heroes quest, even after completion. For shield of arrav you can get lots of half shields (or certificates) by dropping them (or they fixed that, not sure) and in that way you can help other players by giving your duplicates to them. I once (years ago) finished shield of arrav by finding one certificate on the ground