I noticed the sandbox mode's park entrance isn't exactly in the middle of the park edge. I want to have a main concourse going through the middle of the park but it's lopsided as it is presently. Is there anything I can do in scenario editor or whatever to fix it so that it's in the exact middle of the edge of the park?

You can make your own scenario with unlimited money with the entrance in the middle. I also noticed that entrance too, it appears that the people at Atari were lazier than me or they are terrible at keeping something straight.

You can actually open sandbox parks in the scenario editor. Just look for a drop-down menu in the load window while you're in the scenario editor and you should be able to find it.

While in the scenario editor, you can change the entry/exit points and place/remove the park entrance as if it were scenery.