Max carbonation pressure

Do you think there is a level of pressure at which carbonation will stop? If I naturally carbonate in a keg but I also apply CO2 from a canister, will the pressure stop the yeast from doing it’s job?

Yes, under enough pressure, the yeast will stop metabolizing sugars into alcohol and CO2. It’s a really high pressure, however, so your beer will be overcarbonated before the yeast stops working.