At the risk of getting chastised...
1: 30 or even 60 days isn't really an issue. If it's 90 days, you may want to drain it. If the boat runs, run it to half a tank, then fill it with fresh. Even if you drain it, you'll still be burning the stuff in the lines anyway. You won't be doing damage by running 2 month old gas, it just might run a bit rough until it gets fresh stuff.
2: Oil is subjective. It's more important to change it with quality stuff (and a quality filter) at 50 hours or once a year than what you use. Some swear by Shell Rotella or Mobil 1, I like Amsoil 20-50 synthetic. Don't forget to change the tranny oil with Dextron 3 or 4.
3: You'd need to post a pic or go by the serial numbers. The hull number would tell you what's supposed to be in the boat, the engine number will tell you what it is. Sounds like a Chevy 350 efi to me which was an option. See your manual or hit the knowledge center on the Mastercraft site.
4: Yes, I would get the gel repaired. Some people don't worry about it, but I would at least roller (thin foam roller) some gel coat or marine enamel on it to keep it sealed.
5: A new prop can be expensive. You can (and should) get your prop repaired. Those bent edges can cause vibration and affect performance. It's $100 well spent. If you really want to replace it, you can go to a 4 blade for a bit more hole shot.
Some mornings it just doesn't seem worth it to gnaw through the leather straps. -Emo Phillips