The issue was this: Trinity's appearance.2da is based on MotB's version, and the Drider's appearance 2da is based on the SoZ version. And... there is an extra column in SoZ!: animation speed or something. So copying the 2 lines from the Drider 2da to our 2da was causing the error. I had to remove the extra field.
Would be nice if the Driders came with MotB compatible 2da lines (or at least a little note about the need to remove the extra field). And if you do that, may as well delete the repute.fac file from the archive as this too gave me grief for a while.
But now everything is well, and the Driders kick ass in Trinity now! They are a bit weaker than I expected though. I thought they'd be as tough as the beholders

Thanks for trying to help us work out the issue, and thanks for the great models!