Is it possible to avoid the NTR/corruption in the game?
Yes, it is possible to avoid the NTR/corruption. There's an early path that allows you to do so, but the main path involves Hannah being corrupted over time unwillingly due to circumstances involving her husband and another character.
What happens if I refuse to visit the mom in the game?
If you refuse to visit the mom, you'll later catch the guy creeping on her in the library and MC will get angry. The Mom also becomes aware of this situation and seems to be going along with her son's suggestions.
What are the different choices that affect the NTR path?
There are several choices that influence whether you go down the NTR path or not. Visiting the mom in the library and helping Caleb study after it pushes you further into the NTR path, while other choices may avoid it but set up different approaches.