I agree with you that the oral transmission of the Beowulf epic is cloudy and possibly the names are corrupt. Still, there is historical evidence for Hrothgar and, I think, for his older brother who was passed over as the inheritor of the spear Danes. It is tempting to think that Heoroweard was passed over because he had already immigrated to Britain, which had begun in the 500's after the absconding of the Romans in 411. Still, while Hrothgar was a historical person, there is no such historical record of Beowulf himself. Thus, you are right to suspect the veracity of the Heoroweard name.
Coincidentallly, an article recently appeared attempting to show that the author of Beowulf was the AngloSaxon scribe in King Alfred's court, Cynewulf. If true, that would put the writing of the epic at about 900, possibly 3-400 years after the events it describes. And then there are textual variations in the surviving manuscript, which is written in two distinct hands at different times and is apparently a copy of an earlier work, etc. etc. It's a twilight area, but it may be the best we can do as to the earliest origins of the name. Like you, I can't "quite place everything on Heoroweard" either.