Use 1 medium mashed banana to replace the eggs. They will be a little softer so be gentle cooking them.
Yes you can use half the bag. Just half the ingredients (1 egg + 0.75 C of milk) and use a smaller bowl to mix. Reseal your bag of pancake mix and keep in a cool dry place or in the fridge.
You can make the mix the night before and keep it in the fridge in an airtight container. Leave it to warm up a little before using it and give it a good stir.