Now that the rains have returned here, I can now tell if the rain gutter water leak I fixed was indeed the source of my water on the passenger carpet.. the answer is that it was the
entire cause of the water leak as I have not had even a drop on that carpet since.
In case others might be trying to diagnose similar problems, I've also included a picture of the passenger side of the floor. The blue line is where the carpet would get soggy -- there was never enough water to pool, though. The blemish on the interior of the side panel was there when I got the car and, while I never saw water drip onto it, that is pretty much where it was traveling to. I suspect that blemish was also a result of this leak since it was clearly created by something dripping onto that area, although what could cause that but not leave any traces on the inside of the upper panel (which it had to have been running down) nor on the outside of the car eludes me.
Hope this helps someone.