IIRC: Olympic couldn't be insured as it was involved in a collision with HMS Hawkeye; and was found at fault by the Royal Navy. There was also damage that couldn't really be repaired resulting in an permanent slight port list and it was a floating legal liability. Titanic was insured for double its worth after this and White Star was owned by noted scumbag JP Morgan. Olympic was also being repaired at the same shipyard Titanic was being built at, making a switch that much easier.

My opinion: They swapped the boats in the yard to pass regulatory inspection for Olympic, then swapped back if there's any truth to this.

To your last point, the reason was that the Olympic was dealing with some manufacturing issues and couldn’t be insured unless it was basically rebuilt for a new inspection.

Instead of rebuilding it, they secretly renamed it the Titanic so it could be insured fraudulently under the Titanic’s insurance, then intentionally sank it to claim the insurance on it. Now they have a good ship (the real Titanic) which will pass inspection as the Olympic, and they have the insurance payout from the sunken Olympic which shouldn’t have been able to be insured.