Was the -s in Athens originally the plural -s?

In Greek and Latin, some cities, like Athens and Thebes, are pluralia tantum, that is, they are always plural. In English, on the other hand, both names are singular, at least in modern English. It has, however, been suggested that the English -s was originally a plural -s, as a loan translation of the Greek and Latin plurals. Do we have any reliable sources that confirm this? Does it apply to both Athens and Thebes?

The Oxford English Dictionary cannot help us because it does not (normally) list the full etymology of names, nor does Etymonline have etymologies that aren't in the Oxford.

In addition, it would be interesting to know if and when the English names were considered plural, and, if so, when they became singular.

Athens had also the singular version Athene during the Classic era.

Athens city of ancient Attica, capital of modern Greece, from Greek Athenai (plural because the city had several distinct parts).

While the city name was plural from the Classical period onward, Homer uses a singular form: Ἀθήνη (Athḗnē).

An example of singular Athene, from Odyssey 7.80, "Athens with its wide streets":

ὣς ἄρα φωνήσασ᾽ ἀπέβη γλαυκῶπις Ἀθήνη
πόντον ἐπ᾽ ἀτρύγετον, λίπε δὲ Σχερίην ἐρατεινήν,
[80] ἵκετο δ᾽ ἐς Μαραθῶνα καὶ εὐρυάγυιαν Ἀθήνην,

Note that Homer also used the plural form. The possibility cannot be excluded that the singular form was an Homeric invention metri causa.

Also singular Mycene, incidentally with the same adjective, Iliad 4.52:

τὸν δ᾽ ἠμείβετ᾽ ἔπειτα βοῶπις πότνια Ἥρη:
‘ἤτοι ἐμοὶ τρεῖς μὲν πολὺ φίλταταί εἰσι πόληες
Ἄργός τε Σπάρτη τε καὶ εὐρυάγυια Μυκήνη:

The following extract from Quora may traces the plural vs singular story of Athens. The -s represented the plural form, at least as Old English is concerned:

Loan-translations or calques occur when speakers of one language borrow not the exact sounds of a word in another language, but the structure of that word.

• In the case of the place name Athens, the original Greek word Ἀθῆναι Athēnai had the unusual peculiarity that it was grammatically plural, marked by the suffix -ai. This is possibly because the word itself a loan from the non-Indo-European language that used to be spoken there before the arrival of the Greeks, and was a common-noun (of unknown meaning) that referred to multiple things of some kind. Some scholars believe that the Anatolian language Luwian was the substrate language spoken in Greece before the arrival of the Greeks

• This word was then loan-translated into Latin as Athēnae, where the -ae was a Latin plural suffix corresponding to Greek -ai. When this word was in turn borrowed into English during the Anglo-Saxon period, the borrowers were well aware of this grammatical quirk and translated it into Old English as Athēnas, where -as was a plural ending in Old English (and the ancestor of modern -s).

  Later, in the Middle English period, the vowel was lost, leaving just Athens.

  However, note that while this toponym is morphologically plural, it is syntactically singular: verbs that agree with it take singular agreement, as in Athens is the capital of Greece.

Old English suffix -as (for plural nouns)

Etymology:

Perhaps from the Proto-Germanic accusative plural ending -anz, with regularly lost -n- before a fricative, or perhaps from the nominative plural *-ōs, a voiceless variety of the regular ending *-ōz. Akin to Old Saxon -os (Low German -s), Dutch -s, Swedish -ar.

• Nominative and accusative case ending, originally of a-stem masculine nouns, later extended to other nouns.

Descendants: English: -s

Maybe there isn't a reliable source in the English language. In Old Greek and Latin it was plural. It is still plural in other languages (for example, in Russian it still takes the plural verb). The same is with Thebes, was plural in Greek, still plural in other languages, has -s in English. P.S. Cannes, Syracuse, Mycenae, Thessaloniki.