I don't mind any Jayne!muse. I was able to deal with a Cas!muse, I can definitely deal with a Jayne one. :-P

As for getting immune to the killing device.... Well, there's probably some more mechanical ways (which ones depend on how it works, I can't remember about it), but those would protect everyone around, not just the Serenity crew.
Besides, if they are genetically modified to be immune to it.... I guess their creators -and bosses- would want to have a way to protect themselves from it too, so the anti-killing-device-device (::g::) maybe already exist somewhere. ;-)