Yep, running a 6 cell when the plan calls for 8 is like putting a stock VW Beetle engine in a Corvette.

Use whatever glue seems appropriate when you repair. CA is generally more than adequate. You might want to get a small bottle of thin CA to use when piecing things back together. With thin, you can hold the pieces together and drip a little on the joint. Don't forget to reinforce all the breaks with an additional layer of wood that's at least the same type and at least as thick as the original wood, but don't go overboard. You don't need 1/4" plywood reinforcement at every little crack. For balsa repairs, balsa is good, or 1/32" aircraft plywood. Over-reinforcing, and using gobs of epoxy will add a tremendous amount of weight.

Do whatever it takes to get the battery in the plane, and have it balance.

How far back would an 8-cell stick pack extend? To my way of thinking the extra cells will still be in front of the CG, or at the very worst, right on the CG. The hump pack design might make the plane too nose heavy.

You know where the CG is on the wing. Transfer that point to the inside wall of the airplane (i.e. just measure back from the front of the wing saddle opening). If the extra cells are close to that point, it won't affect the CG much.