"I have no basis to judge how frequently the FBI uses the strong-arm investigative tactics mentioned in the op-ed piece."
What is "strong-arm" is highly subjective, although the word implies physical violence or the threat of physical violence, but the reality is very often (BushHog knows it too) local and state personnel are "managed" by Federal agents with the local/state folks doing the "heavy lifting" and "dirty work" for the ongoing investigation. It has less to do with "the size of the case" and more to do with available assets and personnel. A "high profile" case might get more Federal attention, and occasionally you will notice the Feds fading into the background before the case gets fully dumped onto the locals.
In the instant case it couldn't get much more "high profile" than the wife of a former President who is also a current candidate for POTUS.
The simple explanation for Hillary's Walk ..is politics. Pure and simple!
Can the BushHog think of any relatively recent Political Prosecutions in Texas? That's just the flip side of Hillary's Walk. Ronnie Earle is the best example.
Prosecutorial discretion vs. Prosecutorial indiscretion.
In the Federal system judges and us attorneys alike have to focused on appointments for promotions. Commie is a short-timer if Trump gets elected .... so he can't punch Hillary out.... and if he saves her ass he might get a cozy bench someplace from which to retire. He's bought and paid for ..... Had he not been a former U.S. Attorney I would say he's just ignorant on the law. But since he is one, he's just a bought liar....and political hack.
If one couldn't convict on circumstantial evidence of "intent" then 80% of the folks in prison would be on the street. He knows it! All they would have to say is .... "I didn't know"!