I think that any attempt to reconstruct a literal history of the Scyldings would be more fanciful guesswork than fact. There is a (probable) mention of Hygelac (Beowulf's lord) in Gregory of Tours, but while we've found a number of late 5th and early 6th century halls in Sweden, we have no way of proving which one of them was Heorot (if it were even a real place and not a fictional re-imagining of the kind of structure that was common in early medieval Scandinavia). It's very challening to locate individual tribes / people groups in the early middle ages when we have good written sources (most of the scholarship that draws those tidy maps of tribes and boundaries in the early middle ages has been soundly challenged for the number of sketchy guesses and assumptions it makes), and finding the central place around which a group that was named in a poem written 400 years after the fact lived is, unfortunately, not something that can be done with anything resembling historical confidence. Heorot may well have been real, but anyone who points to a specific hall and says, 'it was that one!' is exercising wishful thinking. The best we can say is, 'it would have looked like [this site].'

We do, however, know some of what happened to Hrothgar in the poem after Beowulf went home, because Beowulf tells Hygelac (and us) what happens next.

Hrothgar's hall was saved by Beowulf, and the king went back to the feasts and gift-giving that had made him such a successful warlord. But those gifts were going to come back and bite him soon, and Beowulf wouldn't be there to save him a second time.

Hrothgar and a neighboring big-man named Froda had been fighting for years, but they were ready to end their feud and so planned to marry their children together. Froda sent his son with a delegation to Hrothgar, he married Hrothgar's daughter, and all their men began to party. But feuds are hard to end because, even if men are willing to forgive and forget past wrongs, the treasure and weapons that every man carries tell stories about their histories, and that story is difficult to errase as long as men continue to carry this treasure around with them. Beowulf (fortelling these events to Hygelac), sums it up thus: 'No matter how beautiful the bride [with whom we plan to arrange peace], a killing spear is very rarely turned away from seeking revenge after its lord is killed.'

One of Froda's men brings a sword to the banquet, and one of Hrothgar's old warriors recognizes the sword: it used to belong to one of Hrothgar's warriors who died fighting Froda, and it had been looted off the body and, years later, was carried by the son of the thief. The old warrior grabs a young hot-head and points the sword out to him. 'See that sword? The distinctive ring on its hilt? That was your father's, and it was stolen from his corpse by the father of the man who is wearing it. What are you going to do about it?'

By the end of the night, the young man had killed Froda's supporter, reclaimed the sword, and fled into the night. The marriage wasn't ruined, but the shadow cast by this renewed violence caused it to sour, and Hrothgar's kingdom was engulfed in destructive war.

This is a recurring theme in Beowulf: kingdoms built on treasure and gifts quickly grow and prosper, but they fall just as quickly because gifts and treasure, unlike the steady and unchanging bonds of kinship, are just as likely to start fights as they are to pacify antagonists: treasure is treacherous. Grendel attacked Hrothgar because Hrothgar's feasts - where he gave gifts and treasure to his supporters - hurt the exiled monster's ears; that is, he hated the sound of the communal, social activities from which he was banned. The dragon attacked Beowulf because some of his treasure had been stolen. And Hrothgar's hall was engulfed in further conflict after Beowulf saved it from the monsters because the treasure Hrothgar gave - the treasure that made him a great man in the first place and allowed him to build his great hall - perpetuated memories of violence that his children were unable to control or erase. In the end, Hrothgar's legacy was overshadowed, unlike that of Beowulf whose memory lived on because he chose to take his treasure out of circulation, thereby preventing it from causing future harm, by keeping it with him in the barrow built over his body by the sea.

If one wants to choose the most likely origin for the Scyldings, it must be the Danish Skjöldungs in Norse sagas. In Hrolf Kraki's saga Roar (Hrothgar) was succeded by his nephew Rolf (Hrodulf in Beowulf). This lineage ruled "Denmark" for some time after the events in Beowulf. Perhaps some persons of this family migrated to England, which could explain similar grave finds in SE England. Sam Newton discusses this theory in his book "The origins of Beowulf and the pre-Viking kingdom of East-Anglia". And in HKS Hroar is married to an Anglo-Saxon queen. The rest of the people stayed behind and formed the Danish nation later on.

Regarding Heorot: there was som media coverage in 2013 on the oldest 6th century hall in Lejre on Zealand. That is probably the best candidate for the Beowulf hall. Much more on this can be read in the book "Beowulf and Lejre", by Niles and Osborn.