Singular "they" in Greek?

Suzanne has posted on her bookshelf a facsimile extract from Codex Alexandrinus showing Colossians 4:14-15. She presumably chose these verses because of something rather interesting I noticed in the textual variants for verse 15.

First, here is the text of verse 15 in the fully accented Greek of the UBS text:

Ἀσπάσασθε τοὺς ἐν Λαοδικείᾳ ἀδελφοὺς καὶ Νύμφαν καὶ τὴν κατ᾽ οἶκον αὐτῆς ἐκκλησίαν.

This can be translated "Give my greetings to the brothers and sisters at Laodicea, and to Nympha and the church in her house" (TNIV, italics indicate my emphasis, not supplied words). The acute accent on Νύμφαν shows that this is the accusative of the woman's name Nympha, and not of the man's name Nymphas. In fact the earliest Greek manuscripts have no accents. But in this text the word αὐτῆς "her", which must refer back to Νύμφαν, implies that this is a woman's name and so that this is the correct accentuation.

But this Greek text is uncertain. Indeed it is the reading of only a small minority of the surviving manuscripts, including one of the oldest ones, Vaticanus. Nevertheless, presumably because a change from a man's name to a woman's is considered unlikely, modern scholars tend to prefer the UBS text as given above. UBS gives this reading a C rating indicating that

the Committee had difficulty deciding which variant to place in the text.

The majority of manuscripts, including the Byzantine tradition on which KJV is based, but none of the very oldest manuscripts, read

Ἀσπάσασθε τοὺς ἐν Λαοδικείᾳ ἀδελφοὺς καὶ Νυμφᾶν καὶ τὴν κατ᾽ οἶκον αὐτοῦ ἐκκλησίαν.

In other words, they read αὐτοῦ "his" instead of αὐτῆς "her", and where accented they have Νυμφᾶν with a circumflex accent, indicating a man's name. Hence the KJV rendering "...Nymphas, and the church which is in his house" (my emphasis).

The interesting point comes here with a medium sized group of manuscripts, including two of the oldest ones, Alexandrinus as posted by Suzanne and Sinaiticus, which have the reading (with or without accents)

Ἀσπάσασθε τοὺς ἐν Λαοδικείᾳ ἀδελφοὺς καὶ Νυμφαν καὶ τὴν κατ᾽ οἶκον αὐτῶν ἐκκλησίαν.

That is, they have neither αὐτῆς "her", nor αὐτοῦ "his", but αὐτῶν "their", which like all Greek genitive plurals is gender generic, in the sense that the same form is used for masculine, feminine and neuter. Some of these manuscripts have Νυμφαν unaccented (as I have written it here); some have it accented as a man's name and some as a woman's name. But who or what does this αὐτῶν "their" refer to? This is a mystery. Could it refer to the brothers and sisters in Laodicea and Nympha(s), all taken together? This would be rather strange grammar, and a rather strange situation that one house belonged to all of these brothers and sisters as well as to Nympha(s) who was apparently not one of them.

I am led to a rather surprising conclusion. Someone didn't know whether Nympha(s) was a man or a woman and so didn't know whether to write αὐτῆς "her" or αὐτοῦ "his", and so instead wrote the gender generic αὐτῶν "their". This cannot have been the original author, Paul, who presumably knew who he was writing to. But maybe some early copyist, working with a damaged or abbreviated, and unaccented, original, was unsure whether the person referred to was a man, Nymphas, or a woman, Nympha. So, to avoid making too embarrassing a mistake, the copyist wrote the gender generic plural αὐτῶν "their".

In other words, the copyist was doing just what many of us do in English, using a gender generic singular "they" to avoid over-specifying gender - but not in English, in Koine Greek!

Can anyone suggest any other explanation for the reading with αὐτῶν "their"? Has anyone ever before heard of this kind of construction in Greek?