I just checked the 21 full 6-victim Unchained Magics from our 25 attempts and kill this week and can confirm -- every single time, three healers, three dps. Out of 6 healer and 7 eligible DPS, 3 healers and 3 dps were chosen every time. There are 1716 ways to pick 6 people from 13 people; exactly 700 of those are 3 healers and 3dps, or roughly 41%. Twenty one independent choices mean the chance of this is 6.64042516 × 10-9, which is pretty much zero.

He's not referring to the teleporter. Sindragosa's room has three exits: one from the spider room you came in through, one is the teleporter to the left as you come into her room, and finally an elevator in the position to the right that mirrors where the teleporter is. The elevator leads to the initially-closed gate you see in the room with the Val'kyr mini-boss before Valithria. Instead of hanging a right into Valithria's room, you can go straight to the elevator that takes you directly to Sindragosa. It sounds like after the soft reset after killing the Val'kyr, the gate was opened despite Valithria still being alive. Definitely a bug, but a potentially useful one if it's repeatable.

We don't treat this fight like a race in p3. In our 25 man, we put all melee on the frost tombs 100% of the time. Healers and our 3 tanks are in their own channel to co-ordinate so that the tank survives for every frost breath. They reset their stacks whenever they decide it's time to. Ranged are assigned to dps only the boss, they do not help melee on frost tombs so that they aren't killed too fast. Ranged only need to step behind to reset their stacks every 2nd frost tomb, once they're reset they step out and resume on the boss.

This gives everyone a clear single job to focus on and ensures steady damage on the boss. As a raid leader I just have to remind everyone to re-clump in a central spot so that tomb targets have a clear area to go to.

Our sever has gotten the sindragosa raid weekly quest twice in a row for 25. We've missed it both weeks as we were more concerned with just killing her for the first time and then getting our shadowmourne quest done, but the way to do this is to have all dps channel when she's at <1 million health. She should get up to the 75 stacks required pretty quickly, then you finish her off, having 1 person maintain the channel so the buff doesn't fall off. It's easy when everyone does it for a few seconds rather than a couple people doing it all the time.

After you reach the required amount of stacks they will disappear and a new debuff will appear that allows you to loot the quest item. Thus, you don't have to keep up the stacks all the way until she dies. You can only channel when the boss is below 20% HP, but it should be quite easy as long as melee channels when resetting debuff, and casters when they get Unleashed Magic. Though, obviously it's still a step up from a normal kill so you might not want to do it for your first kill.