Dino, if I'm understanding your post correctly, you're off a bit on a few things - but you are correct that
Prized Amalgam isn't as neat & clean as most cards, when it comes to Missed Trigger.
If you explicitly announce a trigger - i.e., there's no ambiguity about which trigger you're announcing - and then fail to perform the actions associated with the trigger, that's a Game Rule Violation (GRV), not a Missed Trigger (MT). And, the remedies for a GRV don't work quite the same as those for MT. What you described - putting Prized Amalgam back on the battlefield whenever it's remembered - is a remedy for MT, not GRV.
For a GRV - not (correctly) resolving a clearly acknowledged trigger - we could consider it a zone-change problem:
Originally posted by MTR 2.5:
If an object is in an incorrect zone either due to a required zone change being missed or due to being put into the wrong zone during a zone change, the identity of the object was known to all players, and it can be moved with only minor disruption to the state of the game, put the object in the correct zone.
If a Prized Amalgam trigger is, in fact, missed, it falls under this paragraph in the IPG:
MTR 2.1
If the triggered ability is a delayed triggered ability that changes the zone of one or more objects defined when the ability was created, resolve it. … the opponent chooses whether to resolve the ability the next time a player would get priority or when a player would get priority at the start of the next phase. These abilities do not expire and should be remedied no matter how much time has passed since they should have triggered.
For a GRV, returning a Prized Amalgam just in time to be a surprise blocker would absolutely not be “only minor disruption”, so that remedy should be disregarded.
For an MT, the opponent can choose between a couple points in time to return the Prized Amalgam, and thus be able to avoid the surprise blocker.
And in either case, let's make sure the Prized Amalgam's controller didn't choose to forget until a more advantageous moment…
d:^D