The third opening in the tank stopper is in case you need a third line for fueling. ie. If the engine were enclosed in a cowl and you couldn't get to the line going to the carb, you could use a third line and a fuel dot, or one of the available fuelers mounted on the cowl. In your case, ignore it.

Best method for installing the "mounting board" is with velcro. You definitely WILL want to remove it someday. You could also make a mount and screw it in. Remember to wrap the receiver, battery pack and fuel tank in foam.

You certainly need a 'Dremel' rotary tool. Tell your wife she should be happy you aren't buying the band saw and drill press, yet.
Dennis-