There is no guarantee for that. (Some of them are, a lot of them are not! I think nightlies from last year are quite stable. But files generated in nightly are generally incompatible with the stable version and sometimes even with other nightlies.)
Yes (From the top of my head: step support, highlight net in schematic, custom filters in cvpcb, update pcb from schematic, 3dshapes importer, …)