Yikes, it's been a while, but here's what I vaguely recall...

If you take the tk50 drive out of the machine and remove the cover (held
in place with about 4 screws), I think you can get the tape out and then
you can carefully rewind it into the cartridge. If the leader on the drive
that hooks onto the tape is damaged, you'll have to fix/replace it before
it'll work again. (I seem to recall cellophane tape has been known to
work?)

Cable pinouts:
- Vague recollection is that the console 9-pin D has 2,3 and 7 the same
  as a standard 25pin D (send, rcv and signal grnd OR rcv, send, signal grnd).
  A true console cable also had pins 8-9 jumpered, but I think it was the
  Vaxstation 2000 that cared about the jumper and not the MVII. (I do recall
  that the pinouts are not the same as a 9 pin D on a PC.)
- Can't remember the MMJ pinout at all, but I think it's specified in the
  DECstation 3100 Owner's Manual.

Sorry I can't help more (those manuals have long since disappeared), rick