This is a pretty loaded set of questions.
To start, just to make sure you are grounded, the NUCs support two types (or levels) of passwords, System (User and Administrator) and Drive. System-level passwords can protect against unknown users booting your machine and/or changing the BIOS configuration. Drive-level passwords cause the data on the drive to be encrypted and not be decipherable without the password. Since the latter is implemented by the drive itself, someone physically removing the drive from the PC is not going to gain access to its data (without the password).
Now, to the drives themselves and BIOS support. Some M.2 NVMe SSDs support passwords and some don't. Those that do may or may not be supported by the BIOS (I believe it depends upon whether or not the drives themselves support the legacy AHCI command set). In addition - and I believe this more likely with the NUC5's - support may only be offered for a single drive (that connected to SATA0 - with means the add-in HDD, not the M.2 SSD, since it's bay gets the connection to SATA0). As for RAID, as stated, if there is support for only a single drive's password, there isn't going to be any support for this feature.
Clear as mud?