I've spent a fair bit of time combing through our logs.
1) The ideal number of people seems to be 4. This results in an unmitigated explosion of 146k per person. Soaking with more than 4 does not reduce the explosion below this number.
2) Yes. The targeted person does tick the orb and get a stack. The tank also always gets a stack. So you want 2 more people to get a stack to bring the damage down.
3) They don't need to be at the back. They just need 2 people in between them and the fireball.
4) They shouldn't need to run out. The detonation (in our logs) always occurred ~0.5s after the orb target took a tick from the orb. So they could be stacked on others, and the orb would be defused.
All that said, I'm really curious about the "all stack up and only 3 get dots" suggestion, because I don't see that in our logs. I see everyone in the stack getting the dot.
thanks, this is exactly the information i was hoping for. sounds like the best method is to have a triangle with tanks and one tip, and the rest of the raid in two stacks of 4 at max melee range, separated by the width of a healing rain. coincidentally, that's exactly what we did this week and it was a pretty easy 1-shot.
you could even have a spot at the "back" of the healing rain where people could go to drop their stacks.