Is it obligatory to make up the Maghrib prayer missed due to the lengthy Asr prayer during the time of Isha, or is it combined with the Isha prayer?

It is not permissible to deliberately prolong the prayer to take it out of its prescribed time. If a worshiper deliberately prolongs the Asr prayer until the time for Maghrib has passed, they may incur sin for intentionally abandoning the prayer. However, if this is due to waswasa (obsessive whispering), then pardon is hoped for, along with the obligation to strive against these waswasa. If the time for Maghrib enters and the worshiper is still praying Asr, then it is obligatory to perform Maghrib before Isha, and this is not considered a combining of prayers unless combining was intended during the time of the first prayer, with waswasa being an excuse. In all cases, one must strive against waswasa and not give in to it.