This is one of them times when I wish I'd written the loader from scratch instead of basing it off existing code. I don't know why you're getting this behaviour, more worryingly I'm not sure I understand the problem well enough to attempt to fix it.
I know well that an 3d file importer is hard stuff. My congratulation to you for having managed to pack a basically working 3ds loader for Xith. Probably though, the so-called smaller details of these 3d files take the most time to add/correct/implement. :-(
Are you saying the Glied7 should appear as a child of Glied5?
Yes. Glied7 should be a child of the Glied2-group, so I think Glied7 to be a child of the Glied5 TG would be fine.
When in the viewer I rotate or move the "Rumpf" group (= Glied1 group in Xith), the group "Deckel" (= Glied7 group in Xith) is attached to it and moves with it. You could also think of some more sub groups, all being childs of the parent. Like the famous "body - arm - hand" nested hierarchy explained in the Xith3d tutoral (ASE loading).
If so, the current code is in know way set up to handle nested hiearchies like this (at least I don't think so).
Ah well, that explains a lot. :-)