Start with the CG at the front of the spar (1/3 inch forward of where you can feel the rear of the spar) and it will fly. You'll probably end up moving the CG back from there. At the rear of the spar to 1/2 inch behind the spar is where you usually end up liking how it performs.
I have no idea what the instructions say any more... I haven't read the instructions on a Kadet in 15 years.

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I never had a problem with a Hayes brand tank. I have seen knock-offs that have been problematic due to poorer quality. The design is fine.
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The only ways to defuel this tank are through the klunk line or by inverting the aircraft. I never defuel any more... I just connect the muffler line to the carb line with a short piece of tupe or a fuel filter. No drips. You can leave fuel in a good quality tank for months.

Note that the way they ship, the lower of the molded in fuelu line nipples is sealed. If you want a 3-line fuel system you have to drill out the lower nipple.