I'm unclear about the rationale behind disallowing Backstab/Gut from having full effect on stunned mobs who happens to be looking at you.

The context of having a mob looking/aggroed toward you is being misapplied with the knife fighter mechanics. There needs to be a differentiation between having the focus of a target and having a target unable to defend itself to its fullest capacity.

Being caught flat footed/unguarded/unaware with a knife fighter around should be a dangerous situation no matter how that is imparted. Finding the weak point/soft parts of the target should be one of the knife fighters defining aspects. If anything having the focus of a stunned target while you shiv a dirk into its guts should impart some extra psychic/trauma damage and/or extra rage and taunt.

Allowing for a divergence between a mob looking at you and a mob unable to defend fully would open up knife to far more players and play styles.