TarionzCousin
Second Most Angelic Devil Ever
3) And finally, what I need help with: Some undeniably draconic elements, enough to challenge a level 5-6 party, but not be a complete TPK, although if it got them to say "well, we don't have the resources to keep fooling around in here; grab the scale and go," so much the better.
Green dragons can breathe water, so it would make sense if some of the areas were only accessible under water--maybe extremely far underwater / further than someone could swim while holding his/her breath.
If Gax didn't allow kobold servitors to live in the caves, does she have other residents that could double as guardians? Undead seem like the obvious choice, or perhaps a construct she stole and re-programmed.
Finally, if you haven't checked out WotC's 3E Draconomicon, I think you should. It has all kinds of interesting stuff about dragons, including lair wards, running a draconic encounter, draconic spells, etc.