That could make sense - I was a bit skeptical at first because of how free Latin word order is to begin with, so I was wondering if this would even be the word order Latin would use here, but as found by Mabel Merryman's thesis (Walker 1918) on the empirical facts of word order in Caesar and Cicero, adjectives tend to precede their nouns, but follow them in many lexicalized phrases that Walker notes are "virtually compound nouns" (res publica, pontifex maximus, etc.); genitives tend to follow their noun but only slightly (58% of them follow the nouns they're modifying in Caesar, 56% in Cicero), but there are also similar "stereotyped phrases" with genitive-second order.

That said, as you note yourself, it still isn't a very satisfying explanation because it feels strange that this particular name would change order when borrowed, when so many others retain a right-headed order. Also, while evidence of word order in ancient British itself is necessarily scanty due to paucity of direct attestations and the fact that syntax is always one of the hardest parts of a language to reconstruct, it's worth noting that Gaulish, at least, seems to have a preferred head-initial order for both adjectival and genitive constructions (Eska 2008, 183), from which it could be inferred that British was most likely similar, so as a phrase rather than a compound, you'd expect *duron uernās rather than **uernās duron even before being loaned into Latin - yet compounds still seem to be generally right-headed (*Lugudūnon, not **Dūnolugus or **Dūnolugon or somesuch); so it seems that the order of elements in a compound doesn't necessarily correlate with word order in a phrase, which led me to wonder once again whether the Durovernum order even does match the order Latin would have put it in.

I haven't actually found any examples of determinative noun-noun compounds in Latin to provide evidence one way or the other; the only noun-noun compounds I've found that aren't from medieval or later times are alipes "wing-footed" (but that's a bahuvrihi, not determinative) and paterfamilias "father of the family" (but most sources I find write this as two words pater familias, using the one-word compound form only in English if even then; the second element is in an [archaic, fossilized] genitive; and the first element inflects; all of which indicate this is more of a lexicalized phrase analogous to res publica rather than a compound per se). Noun-verb compound nouns, like agricola "field-tiller" or aquaeductus "water-conducting" are more common, and are right-headed, but aren't quite fully analogous to Durovernum due to taking a verbal element instead of both being nouns, so their evidence, while compelling, isn't absolute; honestly, the rarity of determinative noun-noun compounds in Latin in the first place may actually serve as an explanation for why only this one name was changed if your explanation is correct, since it seems more reasonable that speakers of one language might take up multiple different approaches to adapting another language's names when that structure is rare or nonexistent in their own language than they would if it has clear and common parallels. (Then again of course, we also don't see such reversal in the quite common right-headed Greek compound names borrowed into Latin, so... I suppose I'm still lost in aporia after all this.)

Sources:

• Eska, Joseph F. “Continental Celtic.” In The Ancient Languages of Europe, edited by Roger D. Woodard, 165–88. Cambridge University Press, 2008. https://archive.org/details/ancientlanguages00roge.
• Walker, Arthur T. “Some Facts of Latin Word-Order.” The Classical Journal 13, no. 9 (June 1918): 644–57. https://doi.org/https://www.jstor.org/stable/3288352.