If flame sensor will affect Motor assembly's functionality of furnace? How does that work?
There is a chemical added to natural gas (ethyl mercaptan) so you can smell leaks. However, this chemical burns, otherwise you would smell it when using a gas stove -- so it will not warn you of a leak after combustion, which is where the Carbon Monoxide (CO) hazard exists.

You should probably inform company B, and your home warranty company, that their technician seems to be giving advice that could get someone killed and get one or both of these companies sued.

This doesn't necessarily mean company A was correct about there being a crack, or that company B was wrong about there being a dirty flame sensor. I'd suggest getting Yet Another technician in there to check both... or simply assuming that both problems exist and having both fixed; at worst, one of them will be good preventative maintenance.