They only get hydrogen that way because methane is also an energy source. It would be trivial to replace it with water plus external heating if you had some other energy source.

You can get hydrocarbons anywhere, petroleum is convenient, but certainly not required.

The raw material of fertilizer is nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Methane is none of those - it's just a carrier for nitrogen.

Same with pesticides, they are organic molecules so need hydrocarbons, but you can use any hydrocarbon, from trees, grass, whatever. Oil is cheap, so they use it, but it would not be a big problem if you had to use something else.