More or less it's a way to milk you outa some cash. Your recipe itself should put you spot on the alcohol percentage you are looking for. If you were doing a brew that needed an artificial boost you'd probably be getting candi sugar with the recipe anyway. So no I wouldn't think you'd ever wanna do that.
Carb tablets cost more than sugar, and sugar works just fine. So no you'd probably not want that.
I have never needed yeast nutrient for pitchable yeast. I pitch vials, smack packs, and dry packets straight into the carboy no problem. One thing you will use yeast nutrient for is if you are making a starter. That it would be important for. And if you are making a starter you can probably use one pack of yeast.
Go to mrmalty.com and look at the yeast pitching calculator. Pitching two vials of yeast is pretty much always a good idea. I made a winter spice ale that needed 3.5 vials, so I just made a starter instead because my LHBS charges 8.80 a vial and I wasn't about to spend that much.
The truth is that none of those should be an addon. You should already know how many vials you need, if you are making a starter, and what your ABV is gonna be based on your recipe anyway before you checkout.
Priming with sugar isn't rocket science. If you boil it it's not going to contaminate anything.