I'm attempting to piece together an itinerary based upon the following maps:
* Mondego Valley, Oman III, between pp. 352-53
* Last stage of Massena's retreat, Oman IV, between pp. 200-01
* First stage of Massena's retreat, ditto pp. 166-67
* A new map of Spain and Portugal: exhibiting the chains of mountains with their passes, the principal & cross roads, with other details, Nantiat (see link below)
From my travels, my understanding of the topography, and the modern maps available on google, I am fairly certain that the modern road you need to follow away from Celorico is the N.17.
Taking into account there could well have been some re-routing over time to result in the route of the modern road.
I'm fairly sure working from the above will help to delineate a fairly clear, and practicable, itinerary - with some slight deviations - through the settlements of interest through to Ponte de Murcella.
I'll be able to do a bit more work on this over the next couple of evenings.