I'll have to look up a few things, I don't create "custom" device files everyday !!!
However, you say you only read pertition 0, what you might want to try ...
Remove drive from system and then rsf any device files.
Reinstall device, let the server create the devive files, may need an insf -e to help it.
Insert tape, mt -f /dev/rmt/0mnb fsf 1
This will forward the tape "one patition", the n in the mnb say not to rewind, then see if you can access your files.
Unless I misunderstood something, what the device file for the tape is doesn't matter, but the n is important. If the tape device doesn't get the command "no rewind" after you have read your fist part of the tape, it just rewinds, then when you try and read it again, you just read the same data.
Hope this helps,
Martin