> Senators are shown evidence, they are not involved in the process of gathering and vetting that evidence. If this process is interfered with then the conclusions are worthless.

Senators can request additional information on the processes that is shares in closed doors. There is also the intelligence oversight committee that can investigate those actions and bring all the parties involved to actually testify. The fact that not many members of Congress questioned what was presented ultimately rests solely on their shoulders, since they can choose to request more information and open inquiries.