I see that you suggest EF for mythic Reaver, but qualify that if bombs are not an issue, SE is good. I disagree, Iron Reaver has odd timers that make Storm Elemental non-ideal.
Based on my log: the timers are:
Blitz 1: 1:05 (perfect for first elemental), positioned back to tank location around 1:10
Blitz 2: 2:10 (good for second elemental)
Air phase 1: 2:15
Ground phase 2 starts at 3:20.
Blitz 3: 4:25
Blitz 4: 5:30 <--- This is the problem spot. Dropping Storm Elemental at 5:00 means you only get 30 seconds of uptime on the boss.
Airphase 2: 5:35
Ground phase 3 starts at 6:35.
Our raid killed it at 7:38. I saved my Storm elemental for the last ground phase, but I still got nothing from my second Fire Elemental.
He does enrage at 8:30, so if your guild is pushing enrage, you can get a full second fire ele off, but if you aren't pushing 8:00, I'm not sure SE is ideal. I stayed SE only because we had hit enrage once already and thought we would come close again. In hindsight, I would have switched to EF or even LM much sooner.