Shot the laser differently. For example, only make the comet smaller and change it's orbit so that some arbitrary city of earth gets blown up with knock-of effects.

If the AI has a naive "save humans" utility function, I don't see how this advantages it.

Hide a message that is far more difficult to detect than P is able to, but which can be used to communicate with a much later AI.

That kind of trade is indeed a problem, but you'd need to have a dangerous "much later AI" in the first place, which is a very bad thing anyway...

Circumventing physics entirely using advanced decision theories. (counterfactual trade, etc.)

That's a general risk - I'll analyse that later, if this seems to work.

The AI loopholes to send a physical femtobot "through the output channel" and escapes entirely.

The output channel is indeed dangerous - it is not constrained through this method, and needs to be controlled in other ways.