That Gant could successfully make it seem to Ecks and his wife that the other had died via the same car bomb, and that Gant could subsequently go on to marry his wife, and that Ecks would never notice that the director of the organization he worked for was married to his own wife whom he thought was dead, or vice versa, and so forth, seems pretty damn unlikely. There's the whole business of funerals, as well as the fact that they would probably run towards the smoldering wreck crying each other's names and therefore hear each other, among other things.
Sever performs the coupe de grace with the MacGuffin nanobot mounted onto a bullet. A nanobot mounted onto a bullet. Poetic Justice though it may be, wouldn't the bullet be just as likely to kill Gant on its own, without the help of the nanobot?
I imagine it's sort of like writing "Fuck you" on the side of a bomb. A bomb without "Fuck you" written on the side still blows up your enemies just as well, but writing "Fuck you" on the side of it is more satisfying for the bomb-dropper.