After some pondering, think I've figure it out.
Best way to go is to set it up with
--create --level=1 --raid-devices=2
for the initial pair of drives, then
--fail --remove
the rotate-out drive, then
--add
the alternate drive. Now there will be
three drive slots with one "removed" and two
"active".
To rotate a drive off-site
--fail --remove
go to the off-site, swap the drives and on return
--re-add
the rotate-in drive.
This way the 'mdadm' UUID labels will stay on the
drives and 'mdadm' will warn against mistakes
such as trying to use the wrong drive for
a mirror pair. Will always have one "removed"
drive.
The
--grow --raid-devices=1 --force
and
--grow --raid-devices=2 --add
would be used only if a drive fails and needs to
be replaced.
If anyone disagrees please advise.