Point three isn't true; I've had setups with sda1+sdb1 forming a RAID1 pair and sda2+sdb2 forming a RAID0 pair
Point five isn't true for RAID5, where the capacity is (minimum size of devices) * (number of devices - 1)
I would just offer RAID1 as an option, and require selection of two drives on which partitions of the same size are created.
Point three isn't true; I've had setups with sda1+sdb1 forming a RAID1 pair and sda2+sdb2 forming a RAID0 pair
Point five isn't true for RAID5, where the capacity is (minimum size of devices) * (number of devices - 1)
I would just offer RAID1 as an option, and require selection of two drives on which partitions of the same size are created.