Nice! I'm impressed you made it to the Vale with lvl 1 characters on insane mode. How did you avoid/heal damage? Lots of R&R? (rest and reloads)
You might make it through the Vale, although the mummies will give you trouble, but I doubt you can get through the Temple of the Forgotten God; little packs of verbeeg + enough damage to kill your unlevelled tank with one hit = really, really difficult. Also, an unlevelled thief probably does not have enough points in PP to get the rings of free action, so that will make the acolytes much harder to beat.
Happy trails.
PS. I think you can only level up one level at a time in IWD 1. I might be wrong, so let me know if you can "squat" and level up several times in one shot.