Here's a capx I made showing my problem (and I assume also showing my lack of understanding on C2 basics):
Thanks if anybody can point me in the right direction!
PS: I've had no problems getting sprites to pin to their creation instances during other events, such as "On Mouse Click" spawn Enemy->Enemy spawn gun->Pin Gun to Enemy. But it doesn't seem to work for "On start of Layout". I've tried with other Events (like an overlapping invisible sprite, or Compare Time) but even with "Trigger Once if true" at the end of the event it just constantly spawns new sprites every tick... which is a completely different problem I'm having, and which is why I want to use the "On start of Layout" for the gun/helmet spawning+pinning.