Congratulations on winning another ship. If you use the 6ex my advice is to get rid of that battery and in no way attempt to fly with it. Guessing that radio is a 72mhz system and even if totally new in box it could be quite old (I am talking ten or fifteen years) and you have no way to to know. Always bad practice to fly with unknown or used batteries.

Why will the battery not come out?

If that airplane has two aileron servos, Yes you can use a four channel Rx and its only necessary to add a Y chord for the ailerons. I stay away from Spectrum stuff and cannot speak for them so I would not use that rx if it is a spectrum 'parkie' type of rx. You will need full range with the glow trainer.

John