Caulking won’t work on Roofs because Roofs have far more movement than do doors or windows with temperature extremes.
Keep an eye on the inside of the roof rafters now.
If you managed to “move” the leak by slipping in a piece of metal, you now have to question why that roof isn’t shedding the water correctly.
I smell poor installation there.
The siding detail is a dead giveaway.
It itself is done improperly.
What you just described to me personally is a mess. There shouldn’t be any water coming in under your valley. Even if you had driving winds and blowing rain, nothing should be getting in that space at the end of the valley.
Just because you see the water ending up somewhere, it doesn’t mean that you can pinpoint it’s leak unless you see where it’s coming in.
You could have a leak coming in at one part of the roof and it could end up 15’ away from where it’s infiltrating and on top of that, be 5’-10’ to the left or right of where the leak is.
There are so many variables.
I’m telling you, that siding detail and valley detail is wrong. There is only one way to rectify it and that’s to tear it out and have it done right.
If it was right, there’d be no leak now, would there?
Rule #1 on repairing roofs…start with the obvious.