I studied how scvs can attack a morphing creep colony underneath the overmind.
There are two places to morph a creep colony for each overmind. I studied the one which is harder to defend when you do a wall-in.
Here is a result. The red circles represent a position where you can click a creep colony.
top left top right bottom right bottom left
As you can see, 6-7 SCVs can simultaneously attack a creep colony, but when you do a wall-in, only 3 scvs can attack it from your side of the wall.
The time required for 3 scvs to kill a sunk is roughly equal to the time for a full health creep colony to morph into a sunk. So you can surely kill it if you start attacking it before it starts to morph a sunken colony.
You have to send scvs to a correct position before you give them an attack command, otherwise, they often fail to find the position to attack and just stop.
I think someone posted recently that blizzard will just restart rankings for next season, because their team is currently in lack of mappool specialist to prepare a new one. Looks like hail to the ringing bloom and veto system.
This map won't be coming back but thanks to OP for taking the time to post this, we should be encouraging people to make posts like this and not only whining about blizz