sorry for my late answer, I've done a further test with leap 42.2 and 42.3 with all patch available until today applied. In both cases the problem is solved, the system can normally boot with /boot on top of a degraded raid1 and / on top of lvm on top of degraded raid1. in the meanwhile a similar problem comes out, hitting system with EFI boot I opened another bug with all the details: https://bugzilla.opensuse.org/show_bug.cgi?id=1059169