Transcripts to Mueller hearing are here:

Why did Mueller say that going against the olc opinion would be unconstitutional?

I am not a lawyer so I dont understand the reason. I thought that the only body that could rule about the constitutionality of something is the supreme court ?

The department of justice policy about not indicting a sitting president is just the result of a legal analysis by a team of olc lawyers. It has not been tested in court ( as far as I know) .
I find it hard to believe that the executive branch can just draft opinions and declare that part of the constitution. What am I missing ?

Justice department letter to mueller

Trying to restrict what he can say even thought he is no longer an employee. Is it me or does it read like a threatening letter a lawyer for the mob would send ?

Particularly the ending : " i trust this information will be useful to you"...

Who would have standing to refer to the international court of human rights ?

I know the Trump administration will not do anything if their justice department headed by Barr gets lawsuits regarding the treatment of migrants at the border.
I am wondering if some entity would have standing to sue them in an international court of human rights ?

Having international arrest warrants issued for the people that are actively enabling these horrors ( head of cbp, local border patrol chiefs of these centers, etc.. ) may be effective.