1, For compatibility a DVD+R9 should be used, most stand alone DVD players will not be able to play a -R DL disc.
2, While your burner is certainly capable of doing this task, the media you choose is also important. I would recommend Verbatim MKM001 for the NEC drives. Updating to the latest firmware may also be a wise thing to do. Also make sure BookType is set as DVD-ROM.
3, Assuming Adobe Encore is capable of authoring a DVD+R DL compliant file set, then you shouldnâ€™t notice the layer break, perhaps a slight glitch as it changes layers