... so Ironworks never gains any card. ...
That is my central / prime point: why results that in no effect(s), not which other card does it with whatever mechanic. And why is there confusion wether or who get the effect(s).
You can shout a thousand times "Possession does blablabla". It only repeats, when or where Possession interferes with the usual gain process. It does not clarify why Ironworks then gives not the effect(s). Which is because of the implicit If you gain that card ....
I wouldn't say that Ironworks has an implicit "if you gain that card" (with or without emphasis). The "it" in "if it is" just refers to the card that you just gained through Ironworks. Possession and Trader both prevent this gaining from ever taking place, so there simply doesn't exist any antecedent for it.
Programming Analogy because those always make things clearer:
If Dominion cards were little programs, it's like when Ironworks gains a card it assigns the local variable "it" to the card. Then it looks up the types of "it" in the next sentence and gives you a bonus accordingly. When Trader or Possession interrupts the gain it also interrupts the variable assignment, so when Ironworks references "it" again it's referencing an unassigned variable. Here a computer program would get an error except that Dominion has a persistent Try-Except statement where any errors are ignored (a python construct. Basically if the program throws an error then you can specify a way for it to handle it). Now I'm sure that's cleared everything up for everyone.
Another point is that the event that replaces the gain could be completely identical to the original gain event. Consider:
Worthless Trader
When you would gain a card, you may reveal this from your hand. If you do, instead, gain that card.
If you revealed this to an ironworks gain it would still prevent you from getting the ironworks bonus. In fact, the only thing that would change is that you would no longer get the ironworks bonus (someone edge case me).
Also note that the only existing "would gain" cards replace the gain with another gain, but this doesn't have to be the case. They could easily say something like "when you would gain a card, instead, +1 card."
I think I should probably go back to what I'm supposed to be doing now...