Is it permissible to suffice with ablution after a wet dream for prayer, due to time constraints and frequent wet dreams, and is it necessary to change underwear?

It is not permissible to suffice with ablution (Wudu') instead of performing the ritual bath (Ghusl (definition)) for major ritual impurity (Janabah (definition)), as Wudu' does not remove major ritual impurity. If time is short for performing Ghusl, the majority of scholars hold that Ghusl is obligatory even if the prayer time has elapsed, and this is the fatwa (definition) issued. As for sufficing with Wudu' instead of Ghusl, it is not valid, and any statement to that effect is an outlier opinion. The prayers performed in this manner must be made up. The absence of an alternative for underwear is not an excuse for abandoning Ghusl; covering the 'awrah with outer garments is sufficient.