You threw me for a loop with the Erdwall question😊. Wouldn't that instant have to resolve in order to trigger Erdwall's ability? Therefore, if the spell is resolved, then a second clue may be placed as the first part of the spell happens, no?
So here’s what happens exactly, step by step from the moment that Confront the Unknown starts to resolve:
You investigate and get a clue.
This triggers Erdwal Illuminator’s ability, but the trigger has to wait to go on the stack until players would receive priority.
The creature targeted by Confront the Unknown gets +1/+1 for each clue token you control. This includes the clue token from step 1, but doesn’t include the clue token you’ll eventually get from Erdwal Illuminator’s triggered ability since it doesn’t exist yet.
Confront the Unknown finishes resolving, going to your graveyard.
You put Erdwal Illuminator’s trigger on the stack because you’re about to receive priority.
You receive priority. If all players pass in succession without taking any actions, then the top object on the stack (Erdwal Illuminator’s trigger) will resolve.
Erdwal Illuminator’s trigger resolves and you investigate, getting another clue token.