If I make a big batch of banana nut can I freeze them?
You can. Wrap them individually in plastic. Be aware that they go stale even when frozen.
Next time I would recommend freezing the batter in batches. You can defrost the batter, scoop into muffin tin and bake as usual and the muffins will come out even better than if the batter was just made.
I often freeze extra muffins for breakfast during the week; the vacuum sealer comes in very handy. They don;t take long to defrost, and you can split them and pop them in the toaster oven while the coffee is brewing ....
Yes, though I recommend using a vacuum sealer if you have one. Either way, make sure they are tightly sealed, as any areas of the muffin in contact with air will dry out.
You might try freezing your batter in individual muffin liners. I've had good luck with this before, but some recipes are more accommodating to freezing than others (some baking ingredients don't freeze so well).