There are ideas here but I am not seeing a way to make them realistic. Ideally an ad containing any actors would somehow be signed by that actor, but with the amount of different platforms and formats I don't really see a practical solution.

How would the ad get signed and when, and how would it stay valid across placements?

They would not in the same way website signatures do not solve phishing sites, only even worse.

How an authentic video by user mjohm959ga9 showing authentic deepfake of mrbeast help solve this?

because those who fall for it couldn't care less about checking signature