TX. I can't directly answer your two questions, but I've had similar issues with legacy boot. From a little experimenting, it appears to me that the Intel BIOS are fairly buggy/don't play nice with some hard drives in legacy boot. UEFI boot seems to work more consistently.
In my own experience, Intel makes good hardware, but these NUCs have terrible BIOS/drivers that are laden with bugs and therefore they aren't very stable.