I like the execution idea! Somebody just brought up the other day that Anakin felt guilt over killing Dooku unarmed, and seeing Mace about to do the same thing would be something of a real important moment for him. But after all these years of seeing this movie, that never occurred to me. I feel that some emphasis on Mace and what may appear to be immoral to Anakin would strengthen the reasoning for his turn against the Jedi in my mind. Perhaps if Mace doesn't say nice things like, "you'll have earned my trust," but instead seems cold after receiving this information from Anakin might strengthen this perception. If some other Samuel L. Jackson quote could be found that included something like, "I'll take care of this myself," it might help as well. But still, I wish somehow Anakin's own guilt could be brought more to the fore when handling this, but flashbacks and internal dialogue don't work well in Star Wars. Perhaps some Hayden Christiansen line could be inserted when talking about how Palpatine should be tried in court, but his filmography is far smaller than Jackson's and it'd be harder to find that sort of a quote I would think.

The Mace/Snape idea is interesting, but I don't know how you'd pull it off given his relatively minor involvement.