How can the clothing of one who has experienced a wet dream be pure, according to many scholarly opinions, knowing that the emission of pre-seminal fluid often precedes the seminal emission, and it can be copious?

What you mentioned about pre-seminal fluid (madhy) preceding seminal fluid (many) is incorrect, as there is no necessary correlation between their emission. Seminal fluid can be emitted without pre-seminal fluid when desire is intense, such as during a wet dream. Seminal fluid is pure (tahir) according to the most authoritative opinion, and the garment of someone who has had a wet dream is pure and does not become impure merely by doubting the emission of pre-seminal fluid, because the original state is purity until certainty of its emission is achieved.