Re: What position should the weakest defender play
I would not recommend putting your weakest player at second base unless you are of the age group where defining "weakest" is actually a reference to arm strength. At very young ages, then it may be a consideration to put a very good fielder who lacks the arm strength to throw from 3rd to 1st or SS to 1st, at second base as long as she can throw accurate slow throws to 1st.
That said, once you migrate past the issue of being of age to make the longer throws, then the issue develops into being qualified to field and throw quickly, not necessarily hard (there is a difference). If you were to break down the actual mechanics of the fundamentals of proper position play, a right handed 2nd baseman probably has the toughest job on the dirt (other than a left handed SS). "Fundamentals" being defined as jab step, flip toss, knee flip toss, reverse direction overhand throw, etc. Cumulatively, the 2nd base spot has more possibilities of delivering the throw to 2nd, than does SS unless the SS is left handed. As for making the double play, it's still 60 feet regardless if the SS or 2nd baseman is standing on second delivering the ball to 1st, so both SS & 2nd need to have the arm strength to get the job done.
While reviewing Candrea's infielder responsibilities, my cliff notes were longer on 2nd base play than any other position he discussed, so IMO, I'd say the reference to 2nd is way off base (no pun intended).
My practice has always been to put my weakest in either left or right based on the speed of my pitching. Generally, I don't know the number of lefties vs righties on our opponents teams, but I do know the speed levels of my pitching crew. Initially, my slower pitchers have a very good left fielder behind her, my faster ones have a good right fielder backing her up. Profile your opponents batters and make your adjustments with your oufield play.