Is ignoring the fatwa of a شيخ who ruled that a make-up fast and expiation are obligatory for whoever experienced a discharge (مذي or مني) during the daytime in Ramadan due to a phone conversation considered a sin, and is expiation obligatory for me despite my conviction that it was مذي?

The emission of pre-seminal fluid (madhy) does not invalidate the fast. Therefore, there is no need for a make-up fast (qada') or expiation (kaffarah) for someone from whom madhy is emitted while fasting. There is no harm in not acting upon the fatwa of those who believe that this invalidates the fast. The cure for obsessive whispers (waswas) is to ignore them and not dwell on them.