those witnesses aren't relevant just beacuse nunes says they are in a letter.
the biden's could be corrupt to their core, as could Ukrainian prosecutors and all those people on the list. that doesnt make them relevant to the question of whether or not trump abused his power in withholding aid unless the ukrainian government agreed to publicly announce an investigation into biden. again, even if biden was guilty as sin, that doesnt change the dynamic one bit. thats why they arent relevant witnesses.
relevant witnesses would be those privy to the president's decision making, the people on the call, the people who discussed the call, etc. the whistleblower in his own report said he only heard about what happened through other people, so he has no first-hand knowledge of any of this, so his testimony isnt going to be helpful. same with schiff. he has no first-hand knowledge of the president's conversations or decision making. he's not a relevant witness.
whether the bidens/burisma are actually corrupt, whether the ukrainian government officials were actually corrupt, whether schiff and the whisteblower were in contact have no relevance to the claim that trump used the power of his office for the purpose of benefitting himself personally in his efforts to get re-elected. its all a side show.
the cons ution says the chief justice is supposed to preside. in any normal case, is is the judge would determine relevancy of witnesses... so that should be john roberts' call regardless of what cocaine mitch says.