Did Des control when he went back in time at the end of his "travels"? At first it was like he was just waking up somewhere random, but at the end of the epi, when he needed to call Penny, he went right back in time to where he was knocking on her door. The more he does it, the more he's able to control it? Can he still do it now that he has his constant?

I think he was switching with a specific period in his life, Faraday essentially told us this idea by telling Desmond to find him at Oxford. I think it was happening quicker as it was getting closer to the point where he was either going to save himself or die

I think it was plot progression for the most part.

They had to squeeze the story into an hour-length show somehow.

Faraday explained that each time you travel, the length of time you are gone increases. Unless you have a constant to bring you back to your present for good you risk your mind "short circuiting" and die like Eloise and Minkowski.

If you could control when you went back, Minkowski would have just stopped doing so and saved his own life.