Glad to hear you made it back down safely. I had a similar situation a few years back, right after I purchased tundra Toy. Had an odd subtle vibration shortly after takeoff. and immediatly turned back to the airport. Turned out to be the center main bearing was starting to slip in it's race...ie failure was immenent.
However, your loss of EGT/CHT temp was probably a glitch in the instrument as a crack there wouldn't have caused a loss of combustion. Divine intervention?

Now to really scew with you...it dawned on me after my incient, that if I'd done the stupid thing and flown it till failure, my insurance would have paid for the new engine, and any other damage resulting. But since I did the prudent thing, I had to pay for it all out of pocket! Is it me, or is that just wrong? you'd think there'd be some bonus for saving them from buying me a new plane.

Phil