Chainring thickness is not a problem at all. On dedicated 10-speed cranks, the rings are placed slightly closer together (put each ring is the same thickness as 9-speed AFAIK), but the difference in spacing is marginal and normally insignificant. I'd say stick with the 6500 crank and chainrings.

I would first see if you can find a right-hand shifter that will work with the 9-speed stuff. Tiagra would be the best new stuff, or you might get lucky and find a 6500 or 5500 on ebay. Then use whichever of the 2 left-hand levers you have matches the ergonomics/feel of the mis-matched RH lever the best.