1) yes you can use EminentWare in this kind of distributed environment, however it is a little unclear what the connectivty constraints are between HQ and the DSSs.
2) yes, I would install a EW server at each location
3) On the reporting, you can do all the WSUS reporting (including DSSs assuming you are doing rollup with one EW server). If you are not rolling up, then you would probably need multiple EW servers (if I understand your deployment). If you are asking about other advanced reporting like users, groups, registry, files, directories, RSOP, services, device drivers, etc. then you would need to have a EW server at each location.