Kol the heritic didn't give me a key now he's gone, so I cannot visit Kerah in the temple, can I continue without it? Also I cannot get through the tunnel below the serpent pits. I have used the lever, but still cannot get along the tunnel, so I cannot reach the idol. Am I doing something wrong? Thanks

There is a post that metions a hexedit for the heretic key - you miss out on a chat, 2 ankh's and a couple of scrolls - nothing you can't live without (although of course you would prefer to have them).

The serpent holds - just did this the 2nd time on my replay and it gave me more problems this time. First task, kill the slimy filth thing otherwise you are gauranteed not to surface, and you will drown. Even with that thing out of the way, it is sometimes a struggle to get out of the water. Pick your smallest character (I breezed through here the first time, but I had a pixie then) and just keep wriggling through. Hope you haven't got a 6 oomphaz party. When you get through, save again before swimming back out. The hole is small enough that even fish can lock your passage.

Oh, the handle - is that like Paranor and Allanon mixed up?

You said you used the lever & still can't get into the Serpent Holds...you need a key that Kreug the Wizard will give you if you tell him yes you are there to join their sect.
Make sure that the guy talking to him has no gold because he wants 1000 GP at that point.
After you get the Serpent Idol, give him the key back & the other doors will open & you will not have to fight any of the priests (yet) as they think you are one of them.
Watch out for the dowry priests during this time as they will try to make you pay if you let one get you close enough to converse.
When one does corner you, again have a guy with no gold tell him you will pay & he'll let it go that one time.
This just gives you time to cruise around & get familiar with the place & gather a few items you need before having everyone out to kill you.
The Torturers will slap you around no matter what so you will have to fight with them when you first encounter them...also, there is a dowry priest right behind the door where you will meet The Inquisitor and you will be left with no option but to ignore him while you are offing The Inquisitor, thus being labeled defilers of the sect, whereupon you are instantly hated by everyone in the temple.
: )