RAID (originally redundant array of inexpensive disks, now commonly redundant array of independent disks) is a data storage virtualization technology that combines multiple physical disk drive components into a single logical unit for the purposes of data redundancy, performance improvement, or both.
It opens the way for scenarios like: One SD card dies, the other seamlessly takes over without data loss. Alternatively, two cards share data, which in theory nearly doubles the read/write speed (but one card dies, and all data is lost).
RAID requires either a hardware controller, or an implementation in software. Both require kernel drivers to handle this.
Part of what you’re describing sounds like the equivalent of dual bios in PCs, i.e. having a failover chip in case something goes wrong. I’m not sure enough of the android boot procedure to know whether would be possible on android. You’d also need a secondary OS option, in case the problem isn’t the bootloader. Something like MSI were doing with Winki perhaps. The latter part can probably be done, but a failover bootloader likely needs hardware as part of the SoC.
For the power reserve, I think I’ve seen batteries that have a physical button on them that enables the battery to drain slightly further than it would normally allow. I wouldn’t go for a separate built-in battery.