it is A VERY nice discussion we are having here ! :-)
1. I'm not as extremist as you Diocle : yes, nothweastern Gaul has never been "romanized" as southern, but still, maybe in an "hellenistic" way, roman cities have been indisputable centers of civilization, which left a lasting imprint. Augusta Treverorum for instance is more than an oasis in a desert. Population, buildings, institutions, army forces, church, it spread romanity for centuries. Just to give an another example, Britannia, on wich we can all accept that romanization as been very light, well its inhabitants tried to carry a roman way of living for more than a century, even abandoned by Rome, without any troops and suffering strong invasion from almost all parts.
2. Let me say how much i disagree on that precise point !! :-) First of all because we're absolutely not dealing with Romanity as an ethnic concept, but politic. That is precisely, imho, the reason of the roman success for centuries : they didn't care about blood, origins, even beliefs of non-roman people. You want to serve the Empire and accept the Emperor as your FIRST (not your only) god. Welcome ! Give us your blood, your energy and you will be roman. It is even written in the law ! Quite normal for a nation of lawmakers and lawyers as the Romans were.
And let me say that if they is ONE region in the Empire wich embrassed Romanity, it is Gaul. In its way, with its particularism. Gaul has been so romanized that at the end of the Empire, it provide a SECOND senate, an elite wich felt ROMAN AND GAUL. And remember Postumus. Even during this crisis, in Gaul nobody wanted to go back to a "celtic identity". Funny thing, today (and i imagine even more during the fifth century), the celtic population of France is precisely the immigrants from britannia, who came in Bretagne. Do you know how are called the two "populations" of Bretagne, even TODAY ? Les "bretons", the guys who have "celtic" origins, you can find from Brest to Rennes,and.... the Gallo !!!! Yes, even now, in 2016, the non-celtic inhabitants of Bretagne have for nickname the gallo-romans. Can you believe this ? ;-)
And we're not talking about southern France, where EVERYTHING remained Roman until the Rennaissance : language (Langue d'oc, versus the langue d'oil more germanic, spoken north of the Loire), institutions (the Law was TOTALLY from roman origin. I have a master in public law, and one of my first lesson in first year of university was precisly : how roman we are, Frenchmen are the last of Romans, no kidding
), architecture, culture...
Gaul has been romanized, it is a fact, more obvious i can describe here i'm afraid.
3. Zero, you're right. Unfortunately, except for some Justinian units, it is the same everywhere. I think it shows, more than the end of regular units(wich occurs obviously, but imho later), the end of sources. No more historian fan of military details in Gaul (or in Spain, Africa, Ilyria...). Gregory of Tours has no army background, as Hydatius in Spain etc etc...
those units didn't vanished. They evolved slowly.
sorry for my bad english, i hope what i just wrote is understandable !