I have threaded plywood before and it works. I like blind nuts.

I used 1/4-20 nylon wing bolts cut down to a shorter length. I cannot remember what size the original screws were. You could always pry out the blind nuts they provided and install them in the new gear mount.

I can't answer the battery question. I bought a bunch of 4.8 volt 1650 mah nimh packs and use them in everything.

You probably can cut a groove in the bottom of the rudder (brass tubing makes nice grooves) and epoxy the wire in. A small piece of sheet aluminum (soft drink can) could be wrapped around the bottom of the rudder to reinforce it.