Could seasonal allergies postpone an oral surgery?

Maybe. It depends on how bad your allergies are and the type of anesthesia that is going to be given to you. If you have severe symptoms and are going to be sedated blockage of your airway may present a reason to delay your surgery. If only local anesthesia is to be given it shouldn't be a problem except if you allergies are real bad you will feel worse after having oral surgery. Talk to your doctor.
Allergies surgery. Not likely unless you are really experiencing severe allergies. This is a good question for your surgeon/dentist. If you are undergoing local anesthetic surgery then probably not a factor at all, if you are doing conscious sedation or general anesthesia, I would ask the surgeon. Each surgeon is likely to have a different opinion. You should be okay though.
Yes. Yes, especially if you are having general anesthesia. If you can't breathe through you nose, sneezing often, or have wheezing as a response to your seasonal allergy. Make sure you call your oral surgeon's office and report your symptoms. They will advise you. Sometimes will ask you to come in to evaluate you prior to surgery.