I have an idea why lore could be taking so long
On the riot games careers page there is a section for "Research and Development" http://www.riotgames.com/careers/32642
It's basically doing stuff to make other games in addition to league.
What if the reason lore has been taking so long is because they're trying to make a second game which is actually in the world of league, or a number of games in different areas of it? So that instead of reading up on the lore, we play through the lore ourselves.
It's probably not what's going on, but it would explain why all the narrative people have such high hopes for it, and why it's been taking so long.