How does one respond to the Malikis who say that doubt about the invalidation of wudu outside of prayer invalidates it, given that one is certainly obligated to pray and is not absolved of this obligation except with certainty?

The majority of jurists hold that certainty is not removed by doubt. Thus, whoever was in a state of ritual purity (wudu') and doubted its invalidation, whether during prayer or outside of it, his wudu' remains valid, and he should not leave the prayer until he is certain of the impurity (hadath (definition)). The proof for this is the Prophet's (peace be upon him) hadith (definition): "He should not leave until he hears a sound or perceives a smell." Therefore, a sudden doubt does not harm the certainty of purity, and this is the view of the majority of scholars.