Is it permissible to pray the supererogatory (Sunnah) prayers sometimes and omit them at other times, especially if the ablution (wudu) is not immediately nullified after the obligatory (Fard) prayer due to frequent passing of wind?

The Sunnah prayers (Sunan Ratibah) after the obligatory prayer are not obligatory, nor is it obligatory to perform them consistently. So, whoever intentionally leaves them does not incur sin, but they miss out on a great deal of good. If you are able to perform the Sunnah prayer after the obligatory prayer and before your ablution breaks, that is better. And if you are keen on performing them and an impediment arises, such as breaking your ablution, then it is hoped that you will still receive their reward.