Hola
Lugo is beautiful, you'll enjoy a visit. I found a day there was enough.
Camino Inglés end of May - I'd take a sleeping bag as opposed to simply a liner although I'm a bit cold blooded. It can still be cold in the winter.
Elevations compared to the Camino Francés - there are elevation graphs in the Guide. It depends where on the
Camino Frances you wish to compare! There are uphills out of Pontedeume and Betanzos and over the hill to Bruma - but these are all achievable in around an hour.
Splitting the route into 7 stages - many people do this nowadays with Albergues in Neda, Pontedeume, Miño, Betanzos, Presedo, Bruma and Sigueiro. As for a telephone - essential for me when I'm walking on my own. If you mean to call hospitaleros to open Albergues then nowadays over 12000 pilgrims a year are walking this route. I'm sure you'll meet others who will have the means to call the hospitalero if they are not in attendance but why take the risk?
Sellos - the Pilgrims Office asks for two a day on all routes especially within 100 kms of Santiago - these are readily available on the Camino Inglés.
Credencial - You may use the same Credencial to walk onto to Muxia etc. No problem!
Buen Camino
John