You only had the one save, and by the sounds of it, one quicksave? <img src="/ubbthreads/images/graemlins/eek.gif" alt="" />

I have all my old saves, but cleared most of Rivertown and Verdistis before entering the castle, so you would have to redo (or miss) some stuff. I'll see where I was around level 27, or what level I was before entering Verdistis.

You do not need to join the thieves' guild to find Zandalor (though there are other reasons to join). Rob will flag the sewer entrance to the castle treasure room for you, but you can find it on your own.

I played a male warrior, but character appearance and class can be independently changed;
Some more hex offsets...

Someone else discovered the split stack cheat; I just repeated it a bunch of times. <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" />
In addition to cheating, iZakaroN's SaveEditor can be used to reassign stat and skill points to adapt a different save to your playing style.