If madhy (pre-seminal fluid) gets on clothes, and then is followed by mani (seminal fluid) which gets on the bed, does the bed become najis (impure) by that? And does the impurity transfer to the clothes if the bed dries and a person sweats on it?

Urine is impure and invalidates ablution. If it gets on clothing, the affected area must be purified. Semen, however, is pure according to the preponderant opinion and does not render the bedding impure if it gets on it. If a person sleeps on bedding that has been contaminated with urine and then sweats, their clothing becomes impure.