Does this mean something like "would seem to have been proved", "has apparently been proved," or something along those lines? (proved? proven? I'm a native AE speaker, but have been asked to use BE for this text)
The "dürfte" suggests to me that the author isn't 100% certain, or isn't prepared to make the claim that this (the point under discussion) is indeed a proven fact. But I'm not sure!
Any suggestions on how best to convey this in English?
Things that occur to me contain different information, so I'd have to check back with the author. But that's not a problem.
"it is widely accepted that ..."
"evidence overwhelmingly indicates that ..."
"there is broad agreement in the field that ..."
For context, here is a considerably trimmed down version of the passage:
Das ‚Königtum‛ war vermutlich bereits den Indo-Germanen bekannt. Daß [Fachkollege X] glaubt, den Indo-Germanen diese Institution absprechen zu müssen beruht auf einem sprachlichen Argument, das sicherlich falsch ist. Denn es dürfte erwiesen sein, daß es sich bei … um … handelt.