RE: Reys fast progression, the ‘Mary Sue’ complaint.

I thought this was explained and dealt with very elegantly in the sequence within Snoke’s chamber. Snoke’s “Darkness rises… and light to meet it” speech clarifies that as Kylo’s dark side power has grown, the Force has found natural balance by also growing the light side. Snoke had mistakenly assumed this growing power was manifested within Skywalker, but in this speech he makes it clear that it was Rey that was the recipient.