Yes I retract my statement about purples dropping from enemies, never had that either other than receiving them as a major ark fall reward.
I've heard 8 percent chance of legendary in tier 4, 5 percent chance from tier 3 and 3 percent chance from tier 2. Judging by all the lock boxes I've opened, I feel this is probably quite accurate.
I've never seen a purple drop from co op, only as rewards for major arks. And even then, as another stated, these purples are usually nades, I can't remember for certain if I've ever even gotten a purple weapon from a major ark. I doubt it though.