The discount is mostly for giving them your money early, not for buying in bulk. Money is worth more the earlier you get it, because it can be invested. You’re giving them money for the promise of something in the future, with very little knowledge on what that is.
Not usually, no. You can sometimes buy packs of DLCs, but that’s not a season pass —though it may use that name. A season pass is, as the name implies, a pass to get whatever content comes that season (or whatever period).