That would potentially poison him. Human, dog, cat, and generally any carnivore/ omnivore shit is considered "hot," and must be treated before use as manure.
Mix it into compost so that the ratio is 30% droppings or less. This can be used as landscaping fertilizer, but even then it's not recommended to use for food production.
I'm unsure. I know there happens to be a correlation between the species that are (mostly) herbivorous, and their dung being able to be used with food production. I don't know if our gut biome would prevent the use of purely vegan poo. I prefer the sewer as a solution, or a latrine.
Yeah they reference this in the book, it's always from him when he uses the bacteria (meaning in theory it's only his own pathogens or bacteria). He uses his crews shit that's been dried and left in Martian temperatures so it's sterilised.