All of the above. Try thermal, microphone if you have access to one. Check everywhere for the water down low. Is it on a slab or basement? What kind of pipes? Can you not see signs of moisture anywhere? Of course check all under sink and counter pipes first.
Cut out sheetrock in targeted areas...
You really don’t want to end it sooner. You just need to end the flashing such that the water comes out on top of the gutter cover, or the gutter on the lower roof. Not behind the right end of the gutter. And not up against the siding on the right side either.
So ideally I would turn the last...
It looks mostly like Omniwood, Louisiana-Pacific, or Masonite, with a bead. The bead is the little round part at the bottom.
Basically those are all particleboard lap siding, with a bead.
Try something like Pmc Lumber in Marietta.
It could also be a hardy plank made to look like those other...
Agreed. I couldn’t see if there is an end cap on that gutter either. You also need to check the sealant at the endcap. The endcap is basically waterproofed with a gutter specific sealer. Any of that leaking would cause you problems. But it looks like your problem is a little higher up. Step...
The step flashing coming down the wall where the shingles join the wall, should be behind the siding, and out on top of the first layer of shingle. It looks like, but you cannot tell from your pictures, the upper flashing may be OK, but the last piece here in red definitely does not look OK.
Siding should stay a half inch or so off the shingles. But it looks like that lowest piece of step flashing is no good. Looks like it’s running water past the end of the gutter down the siding. Causing exactly the damage you’re seeing. Could also cause soffit damage below.
Probably need to take...