The direct answers to your questions, in order, are:

Yes
As Tina said, backup disk and tape stg pools to a tape copy pool.
Yes.   Move all data off of a volume, delete the volume in ADSM, mirror the
logical volume, recreate the adsm volume.
Yes.  High=0 and low=0 during migration.

If you don't do a backup stgpool to a tape copy pool, you are likely to
loose data eventually.

It is possible to use AIX to mirror your disk pool volumes, but you give up
half your staging area that way.  A better method it to use RAID-5 using a
hardware controller.

I have lost disks in the disk storage pool, but without loosing data.  If
you leave the disk pools unmirrored, leave caching turned on in the disk
storage pool, and force a migration each day to tape of all the data, then
you temporarily have two copies of files until the disk pool area must be
removed.  That protection is insufficient.

If you haven't backed up your disk and tape storage pools, you have one copy
or each version of a file per your copy group policies.  If you loose one
tape, you will loose data.

Mirroring your disk storage pool disks will have a little effect on your
protecting your environment. Copy pools will have a lot of effect.

Bill Smoldt
SSSI