Per the attached video, with the steadfast mutator equipped, you get 10% DR as long as the kilonova buff is active, regardless of whether you are performing a charged melee attack. I didn't include it in the video, but I did test stagger as well and I was able to stagger myself with corrupted meridian while the DR from steadfast was active.
Additionally, I noticed that the darkness beam from a charged melee neutral backdash with kilonova active does not seem to count as a charged melee attack. At least, it does not benefit from the hyper charger mutator's bonus damage or the bonus damage from shield from the shielded strike mutator. This may be intended, but the description of kilonova is somewhat ambiguous, so I thought I'd at least mention it.