Roman Legion in late Antiquity

[Robert Vermaat]

Or recruited from laeti settled on the old territories of the Batavi tribe, perhaps?

Although possible I would say that this is a bit too early for that. When we accept that the palatine Batavi date to the early 4th c. or even earlier, there are not yet Laeti who were settled in the old Batavi civitas.

In the case of the tribunus Batavorum, I still think that he held a higher position that cohort commander. If this was a cohort, he would surely have give the designation (tribunus cohortis novae batavorum, for example). Plus the direct promotion to Dux would be unusual!

Agreed on both counts (although the exact title of a tribune could vary wildly). The guy must have been very well-connected!

More likely that he was either tribune of the mounted guard unit called the Batavi (similar to Aurelius Valentinus, mentioned by M Speidel in Riding for Caesar p.86, who was also given a provincial vir perfectissimus command at the time of Gallienus), or an early tribune of the auxilia palatina Batavi (the seniores/iuniores division had not yet happened at this point). We know that the auxilia were commanded by tribunes - Bainobaudes and the future emperor Valens were both tribunes of the Cornuti.

Both very possible.