#1 Yes, they are good guns
#2 I'd stay with a 12. You can buy 12 guage shells that duplicate 20 perfromance and recoil if recoil is a problem. You cannot get 20 guage shells that will duplicate 12 perfromance.If you want a lighter gun for carrying long distances the 20 makes lots of sense.
#3 No personal experience. I would think they might be a viable option for SD purposes, but not needed for hunting.
#4 Probably not. I do at times use an extended aftermarket extra full tube for turkey hunting. I could probably do just fine with the factory full choke tube.
#5 For SD buy some type of buckshot. I don't worry too much about brand, or even size. I do not recommend smaller birdshot unless nothing else is available. For hunting, there are too many factors to list here. One of the great things about shotguns is their adaptability. You can use a wide variety of ammo to fit the application.
As an all around, do everything gun I like the 24" barrel, especially on a Benelli. A longer barrel would make wingshooting a little easier, but the 24" is certainly useable and is a good compromise if turkey hunting and SD are in the mix. A 24" barreled Benelli is about the same overall length and balance as most other guns with a 26" barrel.