1. They do sell Sikaflex 221 in gray at CCII. That is what AS uses in the manufacturing process nowadays. It is similar to the old Vulkem formulation. Your roof is white, so the C-10 will blend in well. I have used C10 before. I used it on the flat seams around the TV antenna mounting points, the seams of the Armstrong A/C
mount rivets, and the vents. It is self leveling, so it will smooth out to a nice smooth surface.
2. No need to tape it off on the roof seams. I don't think it should be used for any vertical seams like exterior window seams.
3. I think AS uses Dicor lap sealant or some other brand of self leveling sealant.
I would use TremPro 635 on the window seams.
You can get TremPro 635 (the new Vulkem) from Fastenal or VTS, ODMRV et al..it also comes in white if you'd prefer TremPro for the roof seams.
Oh btw, Crestview in Georgetown has C10 as well for cheaper IIRC