Enchantment procs are rppm, so they can largely be ignored (all the extra hits in the world won't increase your enchant uptime--only extra true haste can).
The energy gain from a 2cp rupture would be less than the cost of a mutilate. The poisons range from 10-15sec in duration. The only 15sec ones are wound (you won't be using that) and paralytic (only a 20% chance of proccing instead of 65% for the other nonlethal poisons--of which only crippling lasts 12, which the boss is immune to, so your only bet for a nonlethal is a 10sec poison, meaning deadly is better anyway).
So that leaves only 6 rupture ticks while poisoned (not 7), for an expected energy income of 45 energy. The rupture will cost you 15 energy (it's not a 5-pointer so relentless strikes is not guaranteed), for an overall gain of 30 energy--a little more than half a mutilate.