As far as my experience goes, the healer starts with a Cure 2 -> Regen -> etc., so the initial damage you take is covered by the Cure 2 anyway. Delaying Vengeance isn't the end of the world, I use it directly after my first OP regardless. The reason Flashing isn't ideal is because for those 2 GCDs you spent Flashing for no damage, you could've gotten up Maim and went directly to Overpower town as soon as you're at the end of the pull. Pairing Maim with Unchained + Berserk is ideal of course, but in an AOE situation it's better to press the button that hits all the things for 2 GCDs than buffing yourself on one mob for 2 GCDs. That's why I think drive-by Maim is ideal, the initial and following OPs will get all the hate you need, including all the buffs you'll be popping. Your healer can't heal you as you're going, but they really shouldn't need to. A SS should take care of 75-99% of the damage you take and if you need to you can pop a weak CD like Foresight as you're running.
This still requires 60s between you pressing the Infuriate button and the trash pack you're killing to die, which shouldn't be happening. If it does, your group's DPS is massively slow or you're pulling for a full minute. On the chance that does happen, obviously using Infuriate is good though saving it for the boss could be better, depending on the situation (mobs will probably be low, will probably waste damage on a SC when a BLM is casting Fire2 or w/e anyway).
Yeah, Unchained is definitely way better, but that does assume that there's 2 minutes between pull 1 and pull 2, which there won't always be (especially with a massively geared group and a lower ilvl dungeon). Most newer dungeons (maybe all?) have stop gaps, so you're definitely going to need to not use Unchained on a few of the pulls unfortunately. Thrill + Conv is also good on the boss, yeah, it just kind of varies from dungeon to dungeon and where you need the extra DPS most.
Not sure if 2% extra crit outweights the benefit of having 1 extra Unchained buffed OP, but that's sort of splitting hairs anyway.
Yeah, if you instead go IR + Berserk -> OP -> Thrill + Conv -> SC -> Defiance off + Bloodbath -> OP x inf that would give you 9 hits of Zerk and delay your SC one GCD, it would just cost you one OP with Defiance still on which isn't much of a loss.