Well, everything is pointing towards Necton Robert being yours. I know you want more proof, but it looks like it's just not there to be found.

With Necton and West Lexham being around 6 miles apart it would have been an easy walk back in those days.

Very feasible that he could have been born in Necton and travelled to WL for work. Due to Robert being buried in WL and being OTP of West Lexham when he married, you are looking for a Robert Symonds who was also born there, and there doesn't appear to be one. I understand that you want to be sure you have the correct Robert, but I think you might either have to accept that you'll never find him, or pencil in Necton Robert as yours.
Have you looked at Necton Robert's parents? Do either of them have any traceable links to West Lexham? I don't suppose Necton Robert's father was a mole catcher?
Have you considered things like tithes, taxes etc for a Symonds family living in Necton at the time? (something I have no experience with).
Are there any Symonds families living in Necton and christening children at the time? If so, is there a gap in any of the families where your Robert may fit in (and may have been missed off the register or not baptised?) If not, and if there were only Symonds in WL, this may add a little more weight to Necton Robert being yours.
Sorry I can't think of anything more to suggest. Maybe some more experienced researchers will be able to give some more ideas about where to look ...