1) For reliability from the big three I would say they're all pretty much equal.
2) For gas vs. diesel it mostly depends on personal preference, a diesel will pull a bit better and get better mileage while towing then a comparable gasser. But a gasser will pull whatever it is rated for but at a higher RPM to keep it in its powerband.
3) It used to be true that diesels would get much better MPG then the gassers of that year, but with the new emission equipment on diesels that gap has narrowed to maybe 1-4 MPG stock-for-stock
4) I have no experience around CNG trucks but would imagine that it would be very similar to towing with a gas engine. The worry I would have with that is finding fuel stations that stock it as I know of only one around the area I live that sells it.