Is it obligatory to delay the Maghrib prayer by combining it with 'Isha' (Jama' Takheer) due to frequent flatulence when time is short, or should one pray Maghrib despite repeated invalidation of ablution?

If the emission of gas is continuous such that you cannot perform ablution and prayer, then there is no blame upon you to wait until it stops. If it is possible to combine two prayers by a ṣūrī (formal) combination (by bringing the 'Isha prayer to the beginning of its time and delaying the Maghrib prayer to the end of its time), then that is more cautious. Otherwise, there is no blame in combining two prayers by a jamʿ taʾkhīr (delaying combination). Sheikh al-Islam Ibn Taymiyyah said: "Combining two prayers for someone who has an excuse - such as rain and severe cold wind, and for someone with urinary incontinence or istiḥāḍah (non-menstrual vaginal bleeding) - their prayer with complete purification by combining the two prayers is better than their prayer with incomplete purification by separating them."