If Belaver hadn't sent Neshi, Lenya would have picked up the skull next round (almost posted that for my last action) and tried to UMD it to plane shift us away, since it was pretty likely, that the skull had that power. And winning that combat didn't seem incredibly likely. In fact, surviving more than one round there could have been a challenge already.
1. Return to Temple of Elemental Evil was the first 3.0 module, written before they even finished the core rules. I updated stuff to 3.5 and also incorporated later WOTC books (Feats from Draconomicon for the Dragons, etc.). Some examples of things that worked differently in 3.0 vs. 3.5 -- at various times, you acquired magical items/spells that no longer existed in 3.5. (ring of warmth). Advanced monsters that changed size categories were much more powerful than the initial 3.0 rules (i.e. the Large Gargoyle that slew Neshi's snake form and almost killed Kerwyn, the Huge Howler that killed the drunken paladin NPC).
2. I added a lot of flavor based on Greyhawk or the prior TOEE module, because RTTOE was more of a mechanics module and counted on the GM to provide flavor.