(1) the precancerous cells in the fake meat (which I have no desire to eat) aren't human cells and my understanding is you can't "catch" cancer from the cells of a different species

That's correct. Think about how much trouble we have getting the human body to not kill a donated organ from another human - if the human immune system would just look the other way when you add a piece of cow then we wouldn't have that problem.

On the other hand, it is an extra reason to shoot any degenerate who wants to enable "ethical cannibalism" using vat-grown human meat.