42x13 and 33x23 should be able to use the same chain length. skip the 33x13

also, you could use a double in front and washers on the rear hub to line up one ring with one cog and the other ring with the other cog so that all you'd have to do would be to flip the wheel.

oh, and there's very little chance you'll keep up to your road riding buddies on the descent, unless they're coasting, feathering the brake, and/or not at all tucked.