Permitted routes can be found using the national routeing guide, which can be found at:
http://www.atoc.org/about-atoc/rail-settlement-plan/routeing-guide
There are examples provided and also many examples on this message board, but if you do have any questions do feel free to ask and I'm sure someone will know the answer.
As for the ticket being valid on East Coast services, this should cover that point:
"10. Tickets valid only in trains of particular Train Companies
The validity of a ticket may:
a) be restricted to; or
b) prohibit
travel in the trains of a particular Train Company or Train Companies. Any such restriction
or prohibition will be shown on the ticket."
http://www.nationalrail.co.uk/system/galleries/download/misc/NRCOC.pdf
Assuming the ticket does not have operator restrictions shown on it, and that Newark and Retford is a permitted route, then there is nothing wrong with using an East Coast service.
Also, if one of the online ticket retailers shows something as a valid route, then buy online and you should be fine because of:
"13. The route you are entitled to take
(a) You may travel between the stations shown on the ticket you hold in:
...
(iii) trains which take the routes shown in the National Routeing Guide
(details as to how you can obtain this information will be available
when you buy your ticket)."
As far as I can see, the only such details of how to obtain this information are the suggested itineries.
