I did not find it cryptic. Long. Hard to sort through. But once you found your verse, it was pretty clear.
Why did KJV and Young's literal say 'till? I don't know. Neither one change the meaning.
Lots of context to see what the verse(s) mean(s). It is my strong belief (having not even consulted a commentary or such to get someone else's opinion), having studied the text(s) in Greek and English that the fact that Mary and Joseph had a normal relationship including procreation with one another is self-evident. You disagree. Let the reader decide if there is a case there.
This is a holy image of Our Lady that is most expressive of her essence as the first, pre-Petrine, Church. Note also the three stars, on her forehead and the shoulders. They represent, in the Orthodox tradition, her three "childhoods", that is her natural birth, the virgin birth of her Son, and her sinless life till death. Thought you'd like to know.
The point is that neither Thayer or Liddell-Scott attach any special meaning to the "ou" adverb. The "till" os "until" are valid translations (in my mind, "until" leans more to the meaning you want that "till"). Either with or without "ou" "eos" describes the condition prior to the event controlled by "eos"; whether or not the event changes the condition (e.g, as you argue. St. Joseph no longer abstains from carnal relations with Our Lady) is to be determined by context.