The shortest description of the conspiracy theories that I can think of while retaining the required degree of decorum is that they are a pustulent, foetid, steaming pile of cow dung.
Two salient points support this judgement. Firstly, still visible on the propellers of the stern section at the bottom of the Atlantic is the number 401 cut into the metal. 401 was Titanic's hull number (40909 was bollocks too) and Olympic's was 400. There was nowhere near enough time to swap and rebalance both sets of propellers in the time they had to put Olympic back into service. Secondly, the 1987 IFREMER expedition took a soft wire brush to the encrusted corrosion around the nameplate area on the port side of the hull and revealed the letters "T - I - T - A - N- I - C" physically cut into the plates (see the Expedition Titanic website for the cleaned-up footage). Even if they had removed and swapped the props, there is no way they could physically have swapped the nameplates without removing the fo'c'sle from both liners, de-riveting or cutting the plates away, re-riveting the plates and replacing the fo'c'sle - for which - again - there wasn't even remotely enough time.
That's just for starters - Titanic's fitting out was proceeding apace by the time Olympic arrived in Belfast for repairs and the deck layout was already significantly different between the two sisters. The damage sustained by Olympic, while severe in terms of raw expense, would have been chicken feed in a relative sense compared to the cost of the work these idiots are claiming was carried out, and would certainly not have been enough for Olympic to have been considered a write-off. Remember that in 1907, the White Star approved the salvage of the Suevic by effectively dynamiting the entire bow section and replacing it - this was a far less profitable ship than Olympic, and yet even a repair of that magnitude was considered preferable to writing her off. The Olympic herself received an entirely new stern frame late in her career - the cost of which would have dwarfed the repairs carried out in 1912. Any or all of these documented historical facts should stop any conspiracy theorist in their tracks, but the truth is that conspiracy theorists are almost always more interested in making a name for themselves to propagate or sell their output than any sense of getting to the truth.