Stephen,
You might want to look at the thread 'where (sic) the third class passengers locked up?' (In General Questions).
I think your question is a very sharp one, and is one that I've been looking into for some time. The weight of the evidence is that the mass of third class passengers from fairly early on congregated at, and to some degree were directed toward the extreme rear of the ship. They occupied, for the most part, the public spaces in the rear lower decks (G, F and E Decks). When it comes to these passengers (some 500 of them), from about 12 to 12:30 until the ship went down at 2:20, that's where they were and that is most likely where they died.
You correctly observe that third class passengers (mostly women and children) were not loaded onto lifeboats until around 1:30. Where were these passengers before that--that is, those that did get into lifeboats? Were they in the rear with the rest of the third class passengers and managed (or were led) up to the boat deck? (You might look at the thread a while back on Steward Hart in this regard.) Or were they fortunate individuals that managed to get to the top decks relatively early on (before 1AM) and were never part of the larger group at the rear? If so, they waited patiently (possibly on Deck A) from as early as 12:45 until 1:30, when boats were finally ready for them.
My own opinion at this point is that it is more likely the second of these possibilities than the first. But maybe you'll come to a different conclusion.
Hope that helps.
DG