I watched the current WR, and noticed that the player wins by mercy rule in the first inning, but only does this for bowser. Why would it be faster to not do this for the other characters? Does bowser play for more innings or something?

The game vs Bowser lasts for 5 innings. The other games are 3 innings.

Im currently going through the src page for MSB and cleaning it up, for more questions or info check out https://www.speedrun.com/mario_superstar_baseball/thread/gw737