So right now, at this very moment, who would have the legal authority to order my nephews in the USMC to get on a plane to Iran for combat operations there?
If Congress had authorized operations in Iran, which they have not, then darn near anyone above them in the chain of command. There are documents created authorizing slots for manning. Those are normally signed by the SecDef, IIRC. The positions are funded by Congress, which gives Congressional approval. The individual who fills that slot is normally commanded to go by their CO or by an office that works manning across the service. When I deployed, I was, IIRC, #54 on a list of 57, but I was a volunteer so that made me #1 to go.
Lakin was ordered to go where Congress had authorized and funded operations for years. I deployed to Afghanistan in 2007. Don’t remember who signed my orders, but it sure wasn’t GWB.
Authority to engage in combat comes from Congress. They do not need to declare war, just agree to fund it. That has been ruled as the functional equivalent of declaring war.
The President cannot declare war. He can authorize combat operations for a short time by permission of Congress, which recognizes that we often need a faster response than Congress will give.
See: War Powers Resolution: https://en.wikipedia.org/wiki/War_Powers_Resolution
Also see: https://en.wikipedia.org/wiki/Authorization_for_Use_of_Military_Force_Against_Terrorists