If I were subbing the episode, I think I would end up picking the plural "I love you", too, although it would be a reluctant choice. Picking the plural doesn't effectively communicate the ambiguity present in the English version. However, as you say, I cannot justify using the singular "I love you", because that is reaching in the other direction.
Hey, thanks for your message. :)
Yes, it’s infuriating, isn’t it, because when you translate a text, something inevitably gets lost. It used to annoy me a lot, and then I decided to look at it another way: even for native speakers, there’s no such thing as ‘the same version’ of a text reaching everyone equally. Words have connotations which are informed by our feelings, thoughts and experiences, so we all understand language slightly differently, even when it’s our native language.
That said, yes, this widespread use of the verb ‘to love’ does sometimes make English a challenge for translators, as do the weird undifferentiated pronouns - I had a very interesting conversation with someone a few months ago about the use of the ‘courtesy’ form in Supernatural, and specifically when and how to make the switch from the ‘formal’ you to the ‘friendly’ you when fansubbing or dubbing conversations between Cas and the guys [it’s mostly about Korean - if you’re interested, you can read about it here and here].
Eh, language. There’s always something.