You seem a bit confused about what Senate Democrats can and can't do. They run zero committees, so all of their many subpoena requests have been ignored. They are trying to do their jobs but have been thwarted at every turn by Mitch and the leadership on every committee. Most everything they want to propose never gets to the floor for a vote, because Mitch doesn't want it to. But that's just how the Senate works. Them's the rules, and the majority gets to revise the rules whenever they want.
So what would you have them (or some replacements) do? Go on TV to whine about it and look ineffective? Bring guns into the chamber and start having duels? Fist fights on the chamber floor? Spitting?
Frustrating Democrats is actually Republican policy starting with the Hastert Rule. No matter what Democrats propose, you vote it down or never let it come to a vote at all, no matter how reasonable it is. Note that this has never been Democratic policy! When Democrats hold the majority they are perfectly willing to vote for Republican proposals and sometimes pass them. This is called "normal order". So exactly who makes the Congress not work is pretty obvious.