Mock execution is basically dragging him out somewhere, putting an empty gun to his head and firing it, as a warning (this time the chamber was empty, next time there'll be a bullet). Or something similar to that.

Knowing Michael, he definitely could have made up his mind to kill Fredo immediately; we as viewers are led to believe that the "deciding factor" was Fredo's rant, but who knows. I'm probably giving Michael the benefit of the doubt given how manipulative he was of Carlo in a similar situation, but also remember three things were different between Carlo and Fredo: 1) Carlo wasn't blood, 2) Carlo was beating/abusing Michael sister, and 3) Carlo's actions directly led to the death of his eldest brother and probably helped his father along the way due to the stress.

Fredo's actions COULD have caused problems and certainly set MIchael up for perjury charges, but it didn't result in death of anyone important (I guess for Pentangeli). So I don't think it was quite the same situation as with Carlo, especially with Fredo being flesh and blood not via marriage.