Romanian in the ancient post-Roman chronicles

It seems to me that a vast subject of Romanians re-emerging in the post Roman world in the late Roman written sources has been dropped from consideration. That goes against the well established Scientific Method of investigation that gives preference to phenomena currently observable when interpreting remote ones. If one lists the populations sizes of south Eastern Europe by ethnicity, it would stand as obvious that a Romance speaking population that is at least double in size more than any other should have been just as well represented back during the dark ages. I take full account of that observation in the following part of my answer

I had started from an intriguing phrase in the Gesta Hungarorum, a compilation from the mid 12th century by Petrus notary of King Bela (?) (more Peter Clings's answer to Who were the shepherds of the Romans at the end of the Dark Ages?)

“For after the death of King Attila, the Romans said the land of Pannonia was pasture land because their flocks grazed in the land of Pannonia. And rightly is the land of Pannonia said to be the pastureland of the Romans, for now too the Romans graze on the goods of Hungary”

and wanted to see if there were any other earlier references to the same idea of some Post-Roman people willingly taking land in Pannonia. Surprisingly, I found a similar idea expressed in the De_administrando_imperio from the 10th century as

“The emperor Dioclation was much enamoured of the country of Dalmatia, and so he brought folk with their families from Rome and settled them in this same county of Dalmatia, and they were called "Románoi" ( Ῥωμᾶνοι) from their having been removed from Rome, and this title attaches to them until this day. Now this emperor Diocletian founded the city of Spalato and built therein a palace beyond the power of any tongue or pen to describe, and remains of its ancient luxury are still preserved to day, though the long lapse of time has played havoc with them. Moreover, the city of Diocleia, now occupied by the Diocletians, was built by the same emperor Diocletian, for which reason those of that country have come to be called by the name of 'Diocletians'. The territory possessed by these Románous used to extend as far as the river Danube, and once on a time, being minded to cross the river and discover who dwelt beyond the river, they crossed it and came upon unarmed Slavonic nations, who were also called Avars. The former had not expected that any dwelt beyond the river, nor the latter that any dwelt on the hither side. And so, finding these Avars unarmed and unprepared for war, the Románoi overcame them and took booty and prisoners and returned. And from that time the Románoi formed two alternating garrisons, serving from Easter to Easter, and used to change their men about so that on Great and Holy Saturday they who were coming back from the station and they who were going out to that service would meet one another. For near the sea, beneath that same city, lies a city called Salona, which is half as large as Constantinople, and here all the Románoi would muster and be equipped and thence start out and come to the frontier pass, which is four miles (days) from this same city, and is called Kleisa to this day, from its closing in those who pass that way. And from there they would advance to the river.”

There is section of the Danube that we call in Romanian as Clisura Dunării named that way because it was the section most militarily built up during the late 6th century by the Romans. In Latin the word clausura means "closed entity". It contains at least 10 forts and control towers along a very short 40 km distance along the Danube river where it was easiest to cross. The old Trajan bridge (Trajan's Bridge) was restored for a short period of time and lasted from the reigns of Justinian I to Phocas. That bridge was constantly used by the late Romans on many campaigns as of late 6th century early 7th century, led by Peter and Priscus against the Slavic people and the Avars during the reign of Maurice (emperor). On a map with Roman Roads, the two location would be found as highlighted below

Connecting the old city of Salona to Clisura Dunarii on the google map one can actually see the path of walk between them as suggested by the DEI. It would have been a journey worth taking only one time per year though

The story of the DEI also mentions the Slavonic invasion of Dalmatia following the feat of the Románoi. The abandonment of the old city of Salona (‘which is half as large as Constantinople’) is actually date able to 627 AD which corresponds to the last monument erected. It was not destroyed, but only abandoned, meaning that its population had relocated

Rereading to the passage in the Gesta Hungarorum that says “Slavs [Sclavi], Bulgarians [Bulgarii] and Vlachs [Blachii], and the shepherds of the Romans [pastores Romanorum]”, would it be possible that the Shepherds of the Romans from Salona (aka "Románoi" ( Ῥωμᾶνοι) in the DEI) to have arrived there in 627 AD?

I am thinking that the efforts made by the Románoi to try to reach the Danube and try to defend it must have been motivated by a gain of sort. What could that gain have been?

That can be deduced of course from the type of economy that they were conducting at Salona. Of course, it was shepherding (see the beautiful post Roman medieval monuments so eerily resembling the Roman style at Stećak). They were mostly Shepherds but it was very cold on the highlands (Peter Clings's answer to How could climate have favoured Romanians in the Middle Ages? ) and they were looking for good pastures on the lowlands. Pannonia was to be taken and defended against other competitors. That I believe was the reason for the action described in the DEI

Further indications to possible motivations can be found in the reconstructed climate map below

Why would the Románoi to try to reach the Danube at Clisura and not elsewhere? I believe that the answer lays in the map of the Roman roads. They were well aware of them and wanted to benefit from their usage. They would organize their habitat next to the roads build North of the Danube in the former Roman Province of Dacia. I believe that decision had consequences too for the path set forth of evolution that on the very long run led to modern day Romania a 1350 years later

Could the Románoi have survived in Dalmatia and Bosnia the Middle Ages? They had actually, but in a much diminished numbers. The so called Morlac (Mauro Vlachs) have survived well into the 17th century. Today, there is a fragment of a population in the Istrian Peninsula, some of whom still speak a language called the Istro-Romanian that is related to the Daco-Romanian. The difference in footprint between the two populations is massive, like 10000:1 for the Daco Romanians. That is a very clear indication that the bulk majority of the Románoi left the region prior to the so called “Slavic invasion”. They were replaced by the ancient Croats and Serbians in all habitats. The little pockets of Romance speakers were assimilated during the Middle Ages throughout the Balkans, except for the Aromanians who came South later and moved straight into the Greek Peninsula, never considering Dalmatia as a possible destination again

Could the Keszthely culture be related to the Románoi too? I believe not because the earliest artifacts related to it are date a full century prior. It could be that the Keszthely be one that was created by the Aussonians at the Attila’s court (see the account of Priscus). Although Latin speakers too, those at the Keszthely had tried to manufacture goods and make a living by exchanging them with the Barbarians (Avars). The Románoi established themselves after the Avars had suffered the major blow in 628 Siege of Constantinople (626) that most likely left them dry (population and prestige wise)

Could the army by the Emperor Maurice have been recruited from Dalmatia? It is highly likely for there is a passage in Theophylact Simocatta that describes such the army of Priscus (general) marching against the Avars. At some point in his story a debacle was cause by an individual asking his fellow to look back as he was about to loose some equipment. Those words are assumed as the first expression of the proto-Romanian (only a few generation of evolution away from the common Latin). The word he used are ‘torna, torna, frater’