probably a kill everyone speedrun will take a lot of time to perform, so I expect that this category won't become very competitive. Therefore we could choose just one map for this category, so that the handful of speedrunners that want to try this category can be fully competitive against each other on one map.

I think the Problem would be that someone trusted must watch the whole footage and verify it...

Firstly, this is the wrong leaderboard for such an idea, it would be categorized under category extensions. Second, there's not an actual kill everyone gamemode so there would need to be a lot of extra precautions taken to ensure everyone is competing under the same contract/rules. A category were the run is a contract would be unprecedented for Hitman speedrunning. Lastly, nobody here wants that job or responsibility and we can't just grab some random person to be in charge of it (what Balu mentioned above). I'd recommend just making a google sheet for the couple people interested to submit to.