#erdwal illuminator

The creature will only get +1/+1.

Erdwal Illuminator’s ability is a triggered ability and the trigger won’t go on the stack until whatever triggered it has finished resolving. In this case, that means that Confront the Unknown will be in your graveyard by the time you’re putting Erdwal Illuminator’s trigger on the stack.

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.

No.

While Erdwal Illuminator will trigger when you investigate for Confront the Unknown, that triggered ability won’t even go on the stack until after Confront the Unknown finishes resolving and so the clue from Erdwal won’t exist yet.