Firstly, awesome post.

Really appreciate that you took the time to write out and calculate the bounces  per group, per phase.

We had discussed the tactic of getting two people to bounce it between themselves, but the amount of coordination that would take, may be an issue this early on in progressing the boss.

You did hit the nail on the head though, getting hit by the lines in the intermission is an issue, it does increase the stacks and makes the fight more difficult that it needs to be. I had a hard time last night dodging those lines on some pulls, this is why I was never good at paintball games.

On the healer front. I don't really think it's an overly taxing fight from what we've seen. It's just a case of getting the cool downs in the right order and correct intensity for what we need them to do. Then just spot healing players with OV if they're taking excessive damage.

Looking at the wipes from last night, and reviewing the video before bed, the main reasons seemed to be (and correct me if you think I'm wrong here);
Getting a double/triple stack of the debuff
Additional Add spawning due to soaks being missed/12 stacks
Excessive stacks of the debuff

Now, looking at wipe 8 as an example, the deaths in order go:
Sam - 5:05 (Had 2 stacks upon death)
Me - 5:13 (Had 8 stacks upon death)
Chunks - 5:28 (Had 4 stacks upon death)
Bruce - 5:32 (Had 5 stacks upon death)
(I believe this is the point where I called a wipe).

Lets set a danger threshold of 6 stacks, as that's when the extra mechanics come in, and a warning threshold of 4 stacks.

At the start of the intermission, we have no one above 2 stacks (9 people on 2 stacks, 6 people on 1 stack. - 15 people total. 0 people at any threshold. A total of 24 stacks)

After the first set of soaks we have 1 person on 5 stacks, 4 on 3 stacks, 10 on 2 stacks and 3 on 1 stack. (18 total. 1 person at the warning threshold. A total of 40 stacks [+16]. The debuff itself has jumped once so this should only be +4 stacks )

After the second set of soaks we have 3 people on 6 stacks, 2 on 5 stacks, 3 on 3 stacks, 8 on 2 stacks and 3 people on 1 stack. (19 people. 3 people above the threshold, 2 people at the warning threshold. A total of 56 stacks. [+16]. The debuff itself jumps twice as we hold back before regrouping. So this should be +8 stacks)

In total in the intermission gained us 20 unneeded stacks.

Going into intermission 2. We have gained a further 28 stacks and the fight snowballs from there. The debuff itself has jumped a further 4 times. Which means we gained 12 stacks that we didn't need too.

I know this is a small sample size, as I'm only looking at one pull in response to your post.

For me, there are two issues that need resolving to make this fight easier.
Dodging the lines - which just comes with practice really.
Better raid leading - which again comes with practice.

The reason we left the intermission with the additional stacks is people not dodging.
The reason we went into the next phase with so many stacks is because I didn't tell people what to do with 6+/12+ stacks well enough.

Bouncing the debuff between two people up to 11 stacks, just means it's more risky when they get that 12 stack, as then adds start to spawn and then it does become an almost insurmountable task for healers.

I'm not against giving it a try. But it's a quite complex solution to the problem of dodge the lines