LVM itself does not provide redundancy, that’s RAID. LVM is often used on top of a RAID device. If your boot drive fails, LVM itself won’t save you, RAID (software RAID 1 is really common for a boot drive) can.
LVM can be used to seamlessly move data between physical volumes. You can add a new PV to the VG and move extents between LVs. I’ve used it to love-migrate to a larger drive that way. Once the physical extents have been moved to the new PV, you can reduce the old PV and then remove the old disk.
Correct, however basically no one uses that. The MD (RAID) devices are much more common for that, including under boot drives.
See comparison on ServerFault.