Date of the Gallo-Romance/Northern Romance POD.
IMHO the 2nd century is too late a date for the Gallo-Rmc.-NRmc. divergence. It is important to remember that two dialect areas which remain in contact with each other don’t break, but rather slide apart. Moreover I think we want Germanic phonology to have an influence from the outset, since that’s rather the idea with a substrate: when a language spreads into an area where it wasn’t spoken before the first generation will speak it with a broken accent, part of which will transfer to the native accent of the second and third generations. Also there is no need to assume that all Gallican innovations during the first century spread into Germania.