1) It is depend of JCV version for NT maps. For some device JCV version is not full.
For example in Nuvi 765 - D3084040A.JCV. D2504070A.JCV is big and work good but no free space in device for it (if use webupdater for new map). So i think what Garmin write a support for gmapsup1.img in SW for some device. There is no space for gmapprom.img in this device soon
2) It is analog for CNRussiaNT. There are a two different types a tiles for JCV in file. One for landscape orientation, portrait orientation for the second. So in new device I can see all tiles, in classic - only a half. Analogy - for NTU version all tiles plus new only for dual portrait, in NT only classic landscape.