-According to you, Garth is now fully recovered and back in service as of "Going Boldly."
Which makes no sense to me. He was a killer and a nutcase as well as a shapeshifter. In my opinion, even if he is "recovered" he should still be in some type of nursing home. What if he has a relapse while on duty?
While it was nice to see him, it just stretches the imagination to think he would be re-instated to StarFleet.
It absolutely breaks any line of credibility that Starfleet --let alone Kirk-- would be ok with this after everything Garth allegedly and definitely did prior to and during "Whom Gods Destroy."
For crying out loud, the guy committed genocide. I just don't see any way to rationalize putting the guy back in command of a starship after something like that.