Is it permissible to pray in clothes worn by someone who slept on a bed stained with madhiy (pre-seminal fluid) that was not washed, given the uncertainty about the dryness of the impurity after a day or half a day has passed?

You are permitted to pray in these clothes, and you are not required to purify them as long as you are not certain that they have been defiled. The fundamental principle is that ritual purity remains until certainty of its absence is attained.