Is it best to fully commit to one route for a girl or will you get locked out of some scenes if you don't have enough seduction/corruption because you split the points up?
The choices are basically either corruption or seduction. You will see either of those scenes and get a point (or multiple) in the respective category. Currently you can mix up as you like.
At some point you will be locked into a route. And as already said, just having two savefiles will give you access to all scenes.
When it comes to Side-LIs, those are basically your decision. They only have Affection-Points, if you decide against getting those points with them, you will simply not get their scenes.