No. We have a Preferred and two Power APs, and as long as the Preferred holder is driving you get the parking discount, and as long as they are paying (or at least presenting) you get the merch, dining, and hotel discounts. Just need one AP for those.
It is for us but we go for ~10 days a year. Do a budget for hotel (usually 20% - 30% depending on time of year), parking, dining, and merch and see if it works for you.
Nope, they scan your fingerprint for the first time and scan that in each time after.
All that said, APs are definitely worth it if you go over four days per year. Anything less than that depends on how many discounts you'd end up getting.