.
Every time Durham wants to investigate something, he has to get Garland’s approval.
So, we don’t know how many times he’s been told No already. We wouldn’t know.
We do know that Mannafort, Papadopolous, Flynn we’re all made priority and wrapped up, and sentenced.
We also know Strzok and Page broke the Law.
I’m not sure about needing permission, I need to look into that. But, the examples you gave illustrate the point that someone was really out to get them and used methods not normally employed. Durham, in my opinion, is methodically investigating and not pulling the trigger unless and until he’s got the goods. It also seems as he goes after one, he finds more about others and follows those paths accordingly.