Oof. Yeah, that’s an unpleasant situation.
Normally when I see a double-mounted pool it throws back the “insufficient replicas/corrupted data” rather than “one or more devices is currently unavailable” - I do see some indirect devices there as well, so did you ever do a vdev removal (ie: adding more disks including special vdevs, switching between a virtual and a passthrough disk, etc)? IIRC there was a brief point in time where “block clone exists + vdev removal happens” could cause problems.
Can I get the output of the last hundred or so lines /proc/spl/kstat/zfs/dbgmsg immediately after a failed import?