Is it obligatory to make up the fast if it is highly probable that the water from rinsing the mouth entered the stomach, and the fasting person felt something reach the back of the mouth?

Exaggerating rinsing the mouth is disliked for the fasting person and may lead to water reaching the stomach, which is what happened. If the fasting person exaggerates rinsing the mouth and water precedes his throat, there is a difference of opinion regarding the obligation of making up the fast. The opinion that making up the fast is not obligatory is strong, but the more cautious approach is to make it up.