One of these nails is not like the others

So I had a bit of unwelcome moisture in my garage. This isn't the first time it's happened; I noticed some water on the floor last winter, but it was close enough to the wall vent that I just assumed that some rain had blown in or something. This year I noticed it again and, since there was some stuff piled up by the wall vent that would have gotten wet had any water come in that way, and I observed it to be dry, I decided to have a closer look at the roof.

First up, I stapled a piece of cardboard between the rafters roughly above where the drip was, then waited for it to rain again. The cardboard got nicely damp in a spot directly below the leak, and a bit of investigation revealed a likely culprit:

The nails poking through the underside of the sheathing should all be in roughly horizontal lines spaced about 6 inches apart. There should be a pair of closely spaced nails (tacking down the end of each shingle) and a pair of evenly spaced nails between them (tacking down the middle of the shingles). Having a nail an inch and a half below that line is definitely not expected.

Up on top of the roof, I spotted what looks to be the head of that errant nail.

You can see a dent in the edge of one of the shingle layers (they're 2-layer 'architectural' shingles) just below the tab of the shingle above it, just a little above the image center. I couldn't get close enough to really get a good look at it since the leak is about 4 feet from the edge of the roof and my stepladder isn't quite tall enough to get up on there, but it's the most likely candidate.

So now I need to figure out a taller ladder (the eaves are about 9.5 feet above the ground here, and my stepladder is only 8 feet) and crawl over with a tube of sealant to goop this up. And try not to die in the process.

The good news is that this isn't some sort of systemic problem with my roof that will necessitate a costly and annoying replacement (doubly annoying due to having to temporarily remove and then reinstall the solar panels, which will probably require approval from the leasing company that technically owns them, who went out of business and got bought out at bankruptcy by some random loan/lease servicing company).