Does fasting become invalid if one predominantly assumes water will enter the stomach when deliberately performing a certain act, and water does indeed enter, yet there was no intention to break the fast?

The prevalence of a strong presumption is considered equivalent to certainty by most scholars. Therefore, if you intentionally commit an act that is strongly presumed to lead to breaking the fast, and it does occur, your fast is invalidated, because the instigator is like the direct perpetrator.

You must make up for this day, and the major expiation is not required of you according to most scholars. Your intention not to invalidate the fast does not change the legal ruling. You must repent to Allah Almighty for intentionally invalidating your fast and avoid repeating such an act.