Right, that's how I've also seen costs/effects previously ruled. It seems that, based on this thread, the same extends to triggered actions (e.g. "Each time a minion is played...") as well.
Magic players should be able to fill me in better, but spells on the stack must be resolved even if the originating card is destroyed, right? I don't think LotR ever formalized this.
First of, there's no such thing as a stack in LOTR TCG. The concept of a stack (last in, first out) is
totally different from how stuff happens in LOTR. Again, LOTR and Magic have almost nothing in common, besides being cardgames, so let's completely avoid comparisons between both, specially when discussing such specific issues.
I don't think this has to be formalized; if you pay the cost, you get the effect, unless a card prevents it.
Unheeded doesn't prevent anything, it wounds a minion if it exerts. If that minion leaves play or not, it doesn't matter: a cost was paid and so an effect has to take place. It's easy as ABC. Nobody never said that if a "source" leaves play it's effect won't happen... I've never even seen this terminology in LOTR rulebooks before.