How can one overcome the obsessive whispers regarding the intention for prayer that lead to changing the intention of a Fard (obligatory) prayer into a Sunnah (supererogatory) prayer, and what is the ruling on prayer when silent and odorless gases are emitted?

The cure for obsessive thoughts (waswas) is to ignore them and not pay them any attention. If Satan whispers to you that your ablution has been invalidated, do not pay attention to it unless you have an absolute certainty about which you could swear an oath. If he whispers to you that you have broken your intention for prayer, do not give it any heed. Proceed with your prayer and do not interrupt it, and do not care if anyone enters; rather, focus on your prayer. Do not pay attention to the obsessive thought of breaking your intention or whether you are praying a Sunnah; just continue with your prayer.