The cabbie was at a Mobil gas station, which is located as you noted, in the lower left of the aerial photo (Aspen Hill Rd. and Connecticut Ave.) There is an Amoco gas station, but it is located at Aspen Hill Rd. and Georgia Ave. - which is in sight of GOH cemetary. There is also another gas station (Chevron, I think) at Connecticut Ave and Georgia Ave. - and that is the station I mistook as being the scene of the cabbie's murder - which is also in sight of GOH.
I can't think of any "cover" area in line with the Mobil station where the shooter could have laid in wait for his victim, other than a church lot and pet cemetary on Aspen Hill Rd., almost across from the Michaels store (center bottom and rt of center bottom of your aerial photo); and that line is broken, or almost so, by the Giant food store (large gray roof at bottom left of center in photo). I am not teribly familiar with southern Connecticut Ave (out of photo view at bottom left), but I seem to recall a fair amount of tree cover at both sides of road; not sure if parkland or not.
Now, why cops may have checked roof of Giant makes some sense... Mobil station is much lower elevation than ground where Giant is, and church property directly abutting rear of Giant is even higher elevation than Giant itself. IIRC, church property edge is steep hill from edge of Giant rear, church grounds almost level with roofline of Giant.
Last thing, then I have to get working on signs for Clinton FReep... you are correct that Michaels store is not in shooting line from GOH cemetary, but it is in line with pet cemetary and church grounds (bottom rt and center rt. of aerial photo).
I apologize for giving incorrect information in earlier threads, and made corrective notes on those threads as soon as I caught my error a couple of days ago.