What is the ruling on someone who performs ablution (wudu') while knowing or predominantly assuming that his ablution will be invalidated before the time for the next prayer arrives, and if his ablution is not invalidated, is it permissible for him to pray with it without dispute?

Whoever performs ablution (wudu') with the intention of making prayer permissible (istibahat al-salat) or lifting minor ritual impurity (raf' al-hadath), it is permissible for him to pray as many prayers as he wishes with this ablution until it is invalidated. What matters in invalidating ritual purity is the invalidation of the ablution itself, not merely the knowledge or strong presumption that it will be invalidated. The Companions used to pray multiple prayers with a single ablution as long as they did not invalidate it, as reported in Sahih al-Bukhari from Anas. This is something about which none of the people of knowledge disagree.