Is it obligatory to wash underwear if madhy (pre-seminal fluid) is discharged without knowing if it reached it, or does it remain pure to alleviate hardship?

If you are certain that madhy (pre-seminal fluid) has exited, then it is obligatory to perform istinja' (washing the private parts) from it, wash whatever it has stained on the body and clothes, and then perform wudu' for prayer. There is no need to purify the clothes unless you are certain that the madhy has transferred onto them, because the default state of clothes is purity. If you doubt the exit of madhy, the default is that it has not exited. If you doubt the type of discharge (whether it is semen or madhy), refer to Fatwa no. 118035.