More Merovingian- Kingdom of Soissons

[Flavivs Aetivs]

Me too, but how do you prove it was from a Roman or a germanic in Roman service?

The River Maas Helmet was. The man was a Soldier of Constantine III - not necessarily a Roman, but certainly a Roman soldier.

Drinkwater argues the same thing for the forts on the Rhine and Danube: Archaeological evidence points to these men being Germanic ancestry (mostly Franko-Brugundo-Alemann), but they were "Roman Soldiers" in the professional Roman army, although not "ethnically" Roman.

Short swords yeah, but how do we know it's a semispatha?

True. An educated guess is probably what it is regarding those blades.