I'm pretty sure it depends on your mmr, which is influenced by wins/losses but not directly determined by the ratio. If you win a match against a team of diamonds your mmr is prone to increase significantly more than if you win against a team of bronze players for example, so it's possible to get placed high even with a less than 50% win rate. Turrets/kills/deaths etc don't matter for anything.
LP will decay in plat and above yes. Gold and below no longer suffer from decay.
edit: Half my post was missing for reasons unclear to me when I clicked post. Fixed now.