Does touching women temporarily forbidden in marriage nullify ablution?
It is prohibited to touch, or to shake hands with such women as they are considered strangers, and doing so nullifies ablution.
Does using a gargling medicine break the fast?
If the medicine reaches the body cavity (jauf), the fast is invalidated. However, if it does not enter the body cavity, the fast remains valid.
Therefore, it is advisable to avoid using it during the day in Ramadan.
What is the ruling on using toothpaste during the day in Ramadan?
Using toothpaste does not break the fast as long as nothing reaches the body cavity. However, it is disliked (makruh).
It is preferable for a fasting person to use it before the time of fast starts or after Iftar (breaking the fast) to avoid risking their fast.
What is the ruling on wiping the white area behind the ears instead of wiping the head?
The obligation in ablution is to wipe some of the head; it is not a condition to wipe all of it. Wiping the white area behind the ears suffices for that. And Allah the almighty knows best.