1. It is dependent on the dog and the food being fed. A rough average would be 1/2 cup a day (kibble), but that is highly variable.
2. Just whenever they need it (a constant smell, scooting, weird sitting behaviors, biting back there, etc. Overexpressing isn't good because they start to do it less and less by themselves. If you have any concerns, your vet can tell you if it needs to be done.
4 and 5. Totally up to you on neutering. You can research the benefits and risks (both medically and behaviorally) and come to your own conclusion.
He may have been studded out before. Preop bloodwork is necessary at this age and with an unknown history. The teeth need to be cleaned asap or it could lead to other problems. I use CET vanilla mint toothpaste and a toddler toothbrush along with OraVet.