Sorry, your question deserved an answer before this. You answered most of them though, and did raise a few more.
We assume here that the buildings are on the same circuit from the utility, if not, some answers change. IF they are, then only one transfer switch would be necessary, since if the utility fails, it would start.
If you are using this genset to replace emergency batteries (unit equipment) then yes, you should put a relay in the second building, and could put a second transfer switch in that would transfer ONLY that building. I would view that as optional, since generally the idea behind emergency equipment is to guard against utility failure, and that is covered with the one transfer switch. Realize that unless your doing this in No. Va. , I can't speak for your local AHJ.
Yup, I would say 700-6, 701-7, 702-6 all state that labeling at the service is required.
The relay at the second building (I assume you mean a start relay here) would signal the genset to take the em load of BOTH buildings, but yeah, there's nothing illegal about this, I just think it unneccessary. Again we are concerned about Utility failure, and we should have that covered with the K relays in the xfr switch.
445-10 says the disconnect may or may not be necessary (probably not) but I don't know exactly what type of equipment you have here. The transfer sw. is a disconnect, and you can (probably) shut the genset down.
Clear as mud????????? Good luck.