ZFS - add vs attach, need fix

[MikeP]

Hello,
I thought that I was going about this right, but I didn't.
I need to do this...
I have a 2-way mirror boot ZFS pool, rpool. Both disks are detaching and need replaced. I ordered 2 new disks to replace these (then I'll use the RMA'd disks elsewhere).
The new disk is /dev/sde
I want to make this a 3-way mirror.
I used the "add" command and now have... (This is not the right way to do it.)

NAME        STATE     READ WRITE CKSUM
rpool       ONLINE       0     0     0
  mirror-0  ONLINE       0     0     0
    sda2    ONLINE       0     0     0
    sdd2    ONLINE       0     0     0
  sde       ONLINE       0     0     0

I need to remove sde from this, and then do an "attach" to make sde mirrored inside mirror-0 instead of outside.
Apparently, there is no way to do this, I have to just start all over....

So, the point is a) use attach not add. (At least this is not a production system.)
b) how could I backup the Proxmox itself and then restore it to a new 3-way mirror?
c) if I have 2 SSD and 2 HDD, then should I boot from the SSD or the HDD and use the SSD just for cache/zil?

Thanks.

PS ZFS is awesome. ZFS is powerful. ZFS can let you screw yourself up easily.