"Following Y's instructions" isn't still occurring, and that's the problem. It doesn't say to "replace" Y's instructions with Z, then execute Y's now-changed instructions, it says to ignore Y's instructions entirely and do the entirely unrelated thing Z. Ignoring the instructions and getting +1 Card +1 Action are completely independent events. The former event can't stack, but the latter arguably can.
So, I wouldn't call "ignoring the instructions" an event; I'd call that, like, an event
failing to occur. And I'd say the word "instead" creates a dependency between the event that would have occurred and the event that is to occur in its stead. If the first event would no longer occur, then it has no stead in which another event can occur.
(They
are, of course, separate events in that, like, Ironworks doesn't think it gained a Silver when you use Trader to replace its gain.)
The most relevant comparison is with Swamp Hag. "Gain a Curse when you buy something" stacks, so there's no reason not to think "+1 Card and +1 Action when you play your first Action card" doesn't as well.
The big difference here is that Swamp Hag never uses the word "instead," so it never
replaces one event with another; it just tacks on a new event
after the event that triggered it.
Imagine a Bog Hag that says "the first time any other player would gain a card on their turn, they gain a Curse instead." If you played two Bog Hags and your opponent gained a card, then that effect would
not stack because there's only one (first) gain event in whose stead another event could occur. (Ninja'd.)