So, I understand that you will be between the frontier with Belgium, Lisieux and the walk from Mont Saint Michel, not necessarily in this order. I assume that you intend to do part of this with cars, buses or trains.
If for you it is a kind of pilgrimage, it is (btw, the WWI battle places are not exactly religious places, but they are considered with reverence in France; and I have seen lists of "sons of the parish fallen in wars, defending the motherland" in churches). If you want to register your travels in your credential, do it. It is a personal thing.
There is a specific credential for Mont Saint Michel , the "carnet du miquelot".
I have not been there, but apparently you can get further info in
www.lescheminsdumontsaintmichel.com (but the English version of the webpage is simplified; try the French one). Most lodgments in this way seem to be hostals and hotels (and some families that receive pilgrims) , so I suppose they will not mind your detours, and anyway, your intention is spiritual.
As for the "compostela" certificate (if you are interested) you never know with the staff of the pilgrim's bureau. Theoretically, they only check the last 100 km, but they are sometimes quite unpredictable.
Buen camino!