On that last question raised, not sure if it was answered and while seemingly rare, a criminal case where an investigator (maybe not the lead) did plead the fifth for portions of his testimony was the OJ Simpson case in Los Angeles. IIRC Mark Furhman took the fifth for several questions from defense counsel while testifying. MOOIt's insane.
But will he talk when the defense calls him?
I'm guessing not, because one of the defense attorneys said they'll only need 4 (presumably full) days after the prosecution rests. Proctor is a full week on his own.
Can't say I've ever seen a criminal case where the lead investigator decides to take the 5th.