You're the one grasping at straws, thinking that Dershowitzis the ultimate authority on this. 

Senator Chuck Grassley voted for Trump, and announced on Wednesday that his committee will investigate the circumstances surrounding the firing of James Comey as FBI director and whether there have been any “partisan” attempts to interfere with FBI probes.

Senator John Thune voted for Trump and defends Mueller's integrity adding that the investigation is not a witch hunt.  “...I think it’s better for all of us if that work continues. It’s — obviously he [Mueller] is going to get to the bottom and he is going to find the facts, and I think that’s his role."

Furthermore, Mueller's investigation will go beyond obstruction of justice:  The investigation is examining  any suspicious financial activity between Trump and the Russians. The seeming multiplicity of investigations speaks for itself. 

You can keep trying to wish this away, but it just makes you look foolish.