I'm not sure how Rey being a clone works as a hypothesis, considering a clone by its very definition means Rey would have to be an exact copy of someone else that looks exactly like her.
Throwing my cards on the table, I think Rey turns when the spirit of Palpatine that was trapped on the Death Star 2 corrupts her somehow. Kylo dies, but Anakin's force ghost being so powerful there too after being the place he also died, finally discovers the power to resurrect with Luke's help, and a Skywalker rises in the shape of Ben Solo.
Throwing my cards on the table, I think Rey turns when the spirit of Palpatine that was trapped on the Death Star 2 corrupts her somehow. Kylo dies, but Anakin's force ghost being so powerful there too after being the place he also died, finally discovers the power to resurrect with Luke's help, and a Skywalker rises in the shape of Ben Solo.