Re: 2 point conversion - you need to look at game situations before you can make an analysis like yours. Simple example is you score 3 TD's a game, if you have a 50% success rate, in one game it works out, next game it doesn't. This isn't a decision that can be based on season long stats, it's the game situation that effects most of the decision on whether to go for 2 or not.
Going for 4th and short is also a decision with a huge impact on a game and not to be taken lightly. Obviously, field position plays a big role in the decision, but you can't ignore the momentum swing if you are stopped, sometimes it's best to be conservative. The skill level of your team has a huge impact on the decision as well. If I were Belicheck, I would go for it much more often.