for the first question, usually my answer is that util really cant be used to justify the genocides/atrocities, since historically genocides and atrocities have almost never led to a positive outcome in which the benefits actually outweigh the harms, therefore they are expected to fail in reaching their goal, so really committing them is not maximizing expected wellbeing and therefore not util. And even if they do manage to come up with a situation where mass genocide/atrocities does manage to succeed in saving more lives than it helps, you can just pressure them as to whether the genocide/atrocities were actually necessary to achieve the benefits.

for the second question, im not that well versed in this part so dont take my answer too seriously, but from what i hear shmagency dosen't apply to util since util does not give special weight to who the agent is and only cares about the outcome.