All Gamestudio file formats can change anytime. A change of the WMB7 format is not planned at the moment, but it can happen during the lifetime of A8.

Yes, PVS and BSP data are used by Pro only.

Reading the WMB file sequential or not has probably no effect on speed. The engine does not read the file sequentially. In any case you can skip the legacy chunk and all other chunks that you don't need.