The backstory about Mother Winter being the original and Mother Summer being the second to fill the role wasn't from the books, it was a WOJ in response to someone asking about succession among the Mothers and Queens. His answer was that the first Mother Summer wasn't killed at all, she just got tired of the role and retired, passing on her mantle.
Some people have speculated that MW being the original might mean that the Winter mother mantle is the shard of the original unitary mantle that Hecate chose to keep for herself. I like the theory, but it's unsupported so far.
And as for Lea, the WOJ is that she built her personal power from the ground up, mostly by bargaining with humans to her advantage over the centuries. Judging from Sarissa there's no particular reason an unmantled sister would start with great personal power, but there's no solid reason yet to guess at Lea having a particularly more significant bloodline than any other Sidhe.
The Stone Table may very well have been in play around the events of Hastings, though.