Is it obligatory for a congregant whose ablution becomes nullified in the middle of prayer to immediately stop praying and leave the mosque, and what should he do if there are worshippers behind him and he feels hesitant to pass in front of them?

Whoever verifies that his ablution has been nullified during prayer must interrupt it and leave the mosque immediately. It is recommended for him to hold his nose as a covering for himself. There is no harm in passing in front of the worshipers because the Imam's sutrah is a sutrah for them. If leaving is difficult due to crowd, he should remain seated until he is able to leave, then perform ablution and repeat the prayer.