The idea is original, I'll give them that. The problem is that we are trading setup time for a bigger heal (burst) but Mushrooms are already one of the longest setup time heals in the game.
A way to combat this new setup time would be to allow us to move the mushrooms. There are a few ways you could do this.
1. Give us an ability to move our mushrooms
2. Allow the absorbed overhealing to be moved to a newly placed mushroom (4th mushroom placed would get the overhealing the 1st mushroom had already absorbed)
3. Allow mushrooms to be placed on players
Another way to implement number 2 above would be to have the overhealing buff stack on the druid (but only when mushrooms are placed).
This ability is turning out to be 'guess where the raid is at in 30 seconds' which is not fun.