Hi, sorry to bug you, just excited to find a Beowulf fan. Have you ever looked into comparison between the episodes of Beowulf and other works set in the Anglo-Saxon period? There's theories out there that say Offa and Modthryth are satires on the historical king Offa and his wife Cynethryth. Medieval legends about Cynethryth -- a princess subject to incestuous advances, who is then victim-blamed and sent into exile, locked in a chest set to drift on the sea -- form the backbone of Grimm's Alleleirauh, Penta of the Chopped-Off Hands, and Chaucer's Constance story. In that context, do you think Modthryth's crime could've been killing her father in self-defense when he behaved inappropriately toward her? And then, like in the fairy tales, she's sent into exile (or escapes) and married Offa?
Hey! You're not bugging me at all, I'm always down to talk about Beowulf.
I actually did a ton of reading about Modthryth during undergrad and for my dissertation, and it's a shame that I can't remember any of the specific articles I read. One thing I can say with certainty though is that the dating of Beowulf is subject to huge debate and almost all the arguments about the dating of Beowulf have a lot of merit, so we don't know the context in which Beowulf was composed, and without knowing the time and place of composition it's really hard to say what the real world intentions and implications were.
One theory about the dating of Beowulf is that it was composed at the Mercian court of King Offa, though I don't remember anything about it being a satire. However, something to note is that while mention of the Offa in Beowulf might have been made to appeal to Offa of Mercia, that's not the Offa that Beowulf is talking about. There is a separate legendary figure called Offa. He was a king of the Angles (pre-adventus saxonum, so we're talking Schleswig/Angeln rather than anything happening Britain) who is mentioned independently in Widsith as well as possibly in Gesta Danorum. So whether there was any political motivation behind his inclusion in Beowulf, the legend surrounding him (and likely therefore his wife) already existed and was well-known.
There is also the issue of Modthryth and Cynethryth being two different names. There's a lot of debate surrounding Modthryth's name -- the other two names put forward have been Thryth and Fremu. I only use Modthryth because I think it sounds the nicest and I can't decide which argument I think is the strongest. None of these names are Cynethryth, however. It's very possible that her name is just Thryth in the poem, and if that's the case then it could possibly be a shortening of Cynethrith, but it's important to remember that -thryth is a really common suffix for Old English female names so it could just as easily be a shortening of any other name.
Looking at the later stories you've mentioned, for one thing they are all much much later than Beowulf. They have origins stretching back very far, but the time of recording of all of them is centuries later than even the latest dating of Beowulf. These stories have similarities, but they also have very significant differences and are categorised differently in the Arne-Thompson-Uther index. Allerleirauh is 510B whereas Penta of the Chopped-Off Hands is 706, and the Constance story is only sometimes associated with these.
So generally, I just don't think these one-to-one comparisons work. Looking at the actual text of Beowulf, I also think there is no evidence for this. For one thing, her crime is described in the text itself, and the crime is having men killed for looking at her. I think the only similarities to the tales you have described is the fact that she goes across the sea to marry Offa at her father's exhortation (according to the Bradley translation -- I couldn't be bothered to do my own today alas). But this is a marriage pact between two peoples, those all involve women going to a different land at the request of their relatives, often their fathers. This is mentioned as happening to pretty much every other human female character in Beowulf.
I think basically the confusion here is conflating the origins of a legend with the current meaning of a legend in any given text in which it appears. This is tricky because Beowulf itself shows how much legends can change over time and between cultures. Beowulf mentions Sigmund as the legendary dragon-slayer, whereas Old Norse and German traditions name the dragon slayer as Sigurth/Siegfried. So back to Modthryth, whether Modthryth's legend originated from any of those folklore types of from the same legend that Cynethryth's legend originated from, it's simply not in the text of Beowulf and since her backstory is explicitly described, I think there's no argument that the audience is supposed to be drawing on knowledge of any kind of incest tale or calumniated wife tale. Now whether Modthryth shares her origin with any of these other legends/folk-tales is another, harder to answer question, but my own opinion is that it's not likely since, again, the only information that we have about her to my knowledge comes from Beowulf. I don't think she has anything to do with Cynethryth, whose own legend about drifting in the sea does seem to perhaps link to the Constance story. The thing with Cynethryth's legend is that it was first recorded in the 13th century and so very possibly wasn't even attached to her until after Beowulf's composition.
Tldr; it's all a big jumbled mess but when talking about the actual meaning of a medieval poem, it's always a risk to assume knowledge of any story not in the text. I think Modthryth is exactly as she appears in Beowulf, and I love the idea of imagining her as more justified than she is in Beowulf, but that is an exercise in imagination rather than the "truth", as far as there is a "truth" in legend.
