Fwiw, currently on isotropic you can get draw bonuses and action bonuses and still gain a silver. I'm 9/10 sure I did so.
I think the reason this is so confusing is because it's a memory question.
In MtG, I might have a creature that says "tap: Gain an amount of life equal to this creature's power".
I could use that ability, and with MtG's stack you could kill that creature before the ability happens. So then the question of how much life I need to gain comes up. 0 because the creature is a Nothing-creature now? Any amount I want since there's an undefined value? Or did the game store information about that creature and put it somewhere, so that later when we ask this question the game state knows as well as both human players at the table do that the creature had 3 power right before it died?
In MtG, comprehensive rules explicitly explain that the third option is what happens, and explicitly state that the game state itself keeps a record of what things were like right before they ceased to exist. So in tournament, you gain 3 life. Even so, people's intuitions go both directions on it, and one of the incorrect rulings a judge has given me in tournament concerned Last Known Information. (three fifths of the rulings I've gotten from judges in MtG have disagreed with me and turned out to contradict the official rules, so I'm rather unmoved by Karrow's arguments that Dominion would benefit from top-down rulings and errata)
In this situation, I play an Ironworks. I decide that Ironworks is going to gain me a Great Hall. The way Trader is written, though, it implies that, at least when it is in the supply or BM, gains always wait a moment to see if someone wants to cancel them. So the gain a Great Hall event goes on a stack, or waiting area, or something. Then I get a chance to reveal Traders. I do, so then the gain a Great hall event is replaced with a gain a silver event. That replacement means that the gain a Great Hall event is destroyed, and I stick a gain a silver event in its place. Then I gain a silver, and you go on to the second sentence.
"If it's a green card +1 card, action +1 action, treasure +1$". It, rather clearly to me, refers to the gain a Great Hall event that was destroyed just a moment ago. It's referring to the card gained by Ironwork's ability, and since Traders suggests a delay period just before gains actually happen, it got as far as selection. (you could argue that Traders can swoop in and replace a more general "gain a card costing up to 4$ event", but you'd get the same conclusion I advocate).
However, even though both players at the table know that the card Ironworks is referring to is a Great Hall, the game state itself doesn't know that the card is a Great Hall because the event was destroyed. Dominion doesn't have a last known information rule, so we can't expect it to remember things that have been completely destroyed. There's an exception for when cards imply that some sort of data is being stored, like when Outpost implies the game is counting how many extra turns you're taking so that its last sentence can function. But Ironworks doesn't suggest that kind of exception, the card doesn't function improperly without extra data storage, if you gain a curse with Ironworks it functions the same way that it does when Traders ruins with it so it's just working as intended.
So I agree with Donald X's most recent interpretation. It seems counterintuitive because Ironworks' ability had to get far along enough to know what bonuses it was planning to get, but the game doesn't have a protocol for making decisions based on objects that have ceased to exist, so by default you don't make the decision because you can't.