Can you keep a straight face?

So if the rumors are to be believed, it would seem that Robert Mueller has considered the possibility that Donald Trump could be guilty of obstruction for among other things, firing James Comey.

However, as everyone should know by now, the President had his Deputy Attorney General provide for him an evaluation of James Comey for his review, and then used that evaluation as his primary justification for firing the FBI Director.

As far as the evaluation is concerned, it has been also rumored that President Trump asked his Deputy A.G. to make reference to Comey's lack of public acknowledgement that he (Trump) was not under investigation. This was something that the Deputy A.G. decided against doing.

Now it would seem that the Deputy A.G. would be one of the first people questioned in regards to the Comey firing. After all it would seem that this person would have been one of the last people to correspond with the President regarding the decision that was made, and quite certainly this person would be someone with a particularly important insight to the President's thinking.

So how does a Special Prosecutor such as Mueller justify the idea that he can somehow determine what the President's motivations was, and how important this particular evaluation was to the decision, without actually interviewing the person who wrote the evaluation?

But isn't that the quandary here? Mueller would be derelict in his duty to attempt to make that evaluation without the input of the Deputy A.G.  But yet, he cannot ask this person to provide any testimony or be a witness, because this person is supervising his probe.

Under every conceivable normal, by the books, situation in law enforcement, someone who is not only a possible witness, but someone who is potentially a key witness can not be personally involved in any part of the case. They would most certainly recuse themselves immediately under these circumstances.

But of course if Rosenstein would have recused himself, or if were to recuse himself before the end of the probe, that would put someone else in charge, and likely blow much of the cover that Rosenstein has provided for the expansion of the probe's scope. A new supervisor of the probe might actually put a halt to some of these other "secret expansions" and require Mueller to get back to the original scope of the investigation. A new supervisor would ultimately be responsible for determining how much (if any) of the Special Counsel findings are made public.

Can anyone tell me, with a straight face, that Rosenstein and Mueller are skirting this conflict of interest because it's the most professional and by the books manner of going about these things?