That Thayre that you posted is too cryptic to point either way. If you think it commands "eos ou" being translated as "until" but not "till", why do KJV and Young's literal both have it "till"?
Here is Liddell and Scott on "eos" (see I.6 at link). It says,
6. with Advbs. of Time and Place, he. hote till the time when, c. ind., v.l. for este in X.Cyr.5.1.25; he. hou, f.l. for es hou, Hdt.2.143: freq. in later Gr., Gem.l.c., Ev.Matt.1.25, etc.; he. hotou ib.5.25, etc.; he. pote; how long? ib.17.17, Ev.Jo.10.24; he. tote LXX Ne.2.16 ; he. opse till late, [p. 752] f.l.for es opse, Th.3.108; he. arti 1 Ep.Jo.2.9 ; he. hôde as far as this place, Ev.Luc.23.5.
The emphasis is mine, follow to link for further links. It covers your case, eos followed by adverb ("ou"). It does not fall under the next case in LSJ, indicating "how long?". There is no mentioning of the condition necessarily stopping after the time indicated.
My bet is, if you look up the usage of "eos" and compare it to "eos ou", you will discover that "ou" or "an" simply control the following verb, and no proposition is needed when "eos" controls a noun. Had St. Matthew said "till the birth" rather than "till she gave birth", there would be no "ou". It does not introduce any semantics.
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.