I hope and pray you are correct. Unfortunately, the President has a lot of power to direct troops where he likes. Congress can do little more than jerk funding.
If the President wants, he has all the authorization he needs in the Sep 18 Resolution. As you see how it was applied in the Iraq Resolution, it is clear that it was broadly interpreted. There would be no legal basis against this President if he used the Sep 18 Resolution.
Now, could he prove Al Qaeda or particular person ties to Iran?
I think it would be fairly simple. I'll also bet they've been putting that case together for better than 4 years now. Iran, not Iraq, has long been considered the leading supporter of International terrorism.
Did you read the resolutions I posted?