Richard - Have you tried doing the Checkin without "devtype=3590"?
          That keyword is defined as being for 349X libraries and so
it's uncertain what its effect is in conjunction with a plain 3590.
I presume that you've been able to mount and exercise tapes using the
device-driver-provided tapeutil command as a diagnostic?
When you ran the dsmlabel, I'm curious if you used -search and if that
worked (in that Checkin should be similar).  That is, autoloading did
occur via the stacker as opposed to having fed one tape at a time
manually.
We have a 3494 with 3590s so I'm not familiar with the situation of a
3590 with a stacker.  In particular I don't know how ADSM regards
Private vs. Scratch when the concept of Category Codes does not apply
to a stacker type of library, so it's not clear what Checkin is actually
doing for Status values when no codes are definable for the library.
    Neither the ADSM doc nor the handoff Device Drivers manual says what
to do for managing this device type.  This may be one of those situations
where you have to cautiously try various things until you find what works with
this configuration.                      Richard Sims