What is your setup ?
I am using AHK as my broadcasting software, with DBM, OMEN and recount addons. Grid was disabled since Blizzard's version is good enough for me. I have my own mouse broadcasting scripts over the unit frame.

The team consists of
1 tank - DK
2 healers - Holly Pally and Priest
3 dps - Lock, Mage and ele Shaman


- are you using two healers ?
- one priest for PoH Spam, and another for mouse-over healing ?
Yes, I am using 2 healers - a holy priest for aoe heal and a holy pally for tank healing. Both used the healing over unit frame method. I am very careful over mana usage efficiency. POH spamming may result in mana issue. I may be wrong tho, lol

I prefer cast sequence of COH, holy nova, flash heal/binding heal x 2, then poh., but it's all situational. In the video, Medicci is doing the healing, and her has different healing way.

Do you think it is doable with only 5 toons ? DK(tank), Arcane-mage, Destro-lock, Elem-Shammy, Disc-priest ?
I think 5 toons vs Marrowgar is possible, but single healer setup is very very very dependent on your luck. If you healer get spiked, and few big hits land on tank, then it's game over. Also, I am not sure whether disc priest can fill the role of tank/aoe healing or not for this boss, and over a period of 3-5 minutes? Dun think the ele shaman can heal your tank and spiked toon. Switch to resto is a safer method.

Anyhow, good luck! Let us know the outcome. :-)