I missed the Shep/Grunt/Wrex convo because I was trying to be too clever when I gibbed the save file.
In a game where I successfully sabotaged the genophage cure and got away with it originally, I went back in and switched Wrex to being alive, completely forgetting that Wrex is smarter than Wreave, and thus figures out your deception. Confrontation ensued, Wrex dead. Doh!
I could do it again, but I also missed Kasumi so I need to go back and do things again anyway. I'll get Wrex to the party in that game I think.