good question. this is the million dollar question I too would like to know the answer to.
I have been told, but can not confirm " the x86 systems will only boot from the primary disk slot, so if c3t0d0 fails, then you would have to manually move c3t1d0 into c3t0d0's place to boot the system." Apparently it is a PC thing. I do not know if it is true or not, but as with you, I think it is very important and I too need to know the best practices for replacing a failed primary root drive that is mirrored with SVM.