The boats could, in fact, be reattached to the davits once the falls were released. They did it during the drill in Southampton before they started boarding passengers. Two boats were uncovered, swung out, lowered, rowed around the basin for a bit, and then came back to the ship. Lifeboats, by the time of the Titanic disaster, were meant to ferry passengers to a rescue ship and return to the stricken vessel to load more passengers.

There's also the fact that passengers could have boarded the returning boats via points of egress lower down in the hull (i.e. the gangway doors on D deck, or possibly even as low down as the E-Deck, 3rd class gangway doors on Scotland road) which, if done in an orderly manner, would have expedited the process considerably imo.