Would it be reasonable to retroactively apply timing from the longer runs to the shorter categories? The longer categories medium and hard include the other categories as part of their run. So if I ran medium and my time for the first half beats the easy time, should it apply?

There is only one difference in play, and that's AI strength. This should be a disadvantage though, and make runs of just easy/medium typically faster than medium/hard.

The reason I'm suggesting this is that it would help fill out the categories for a less run game, and stop the first run of easy/medium anyone does from being an automatic record. They should at least learn the game a bit.