What is Wilder getting at when he says that the future is easy to predict--it's the past that's difficult to interpret?
I think he is saying that it is easy to say what can possibly come in the future, because it has not happened yet, but it is hard to understand why things happened in the past, because humans made choices to start wars and such. Meaning there are too many variables to make saying why something happened in the past. Am I on the right track.
Your answer is a good start at making a rational explanation of this line, yes. However, remember that the play is an absurdist comedy, so part of the meaning of the line is that there is no meaning: it is absurd, and the opposite of what is real.
Along with that, though, yes, I'd say you are on a right track. I'd add that the future is largely devoid of emotional connotation, because it hasn't happened yet. All things are possible, and not very real, so it is easy to manipulate them. However, the past is emotionally charged. People care about where we came from, who we are, etc., and that makes it tough to understand, let alone interpret.