How do I break the chain?

I'm not sure I can answer your question. Your game is pretty complex, and while switching to hierarchies has probably made things a bit cleaner and more organized, it's still a complex system.

All I can do is share how I would probably approach this task:

1. Pick all moleks that are still in the hierarchy (i.e. children of Chemic). First, set attached = false for all of them.
2. For each molek in the hierarchy that is directly overlapping (touching) Chemic: set attached = true.
3. For each molek in the hierarchy with attached=false: check if it’s overlapping another molek with attached=true. If so, change its attached to true. (You might need to use a family: “Molek is overlapping MolekFamily.”)
4. Repeat previous step until none such moleks left.
5. Any remaining moleks in the hierarchy that still have attached=false at this point have broken off from the chain. Remove them from the hierarchy and launch them away.

This helps. How would Step 3 look like?

You'll need to add Molek sprite to a family, and move instance variables from the sprite to the family level.

Then you will be able to pick both in the same event, something like this:

Chemic pick children Molek
Chemic pick children MolekFamily

 Molek is attached
 MolekFamily is NOT attached
 Molek overlapping MolekFamily 
 ----> MolekFamily set attached to true

You'll need to add Molek sprite to a family, and move instance variables from the sprite to the family level.

Then you will be able to pick both in the same event, something like this:

> Chemic pick children Molek
Chemic pick children MolekFamily

Molek is attached
MolekFamily is NOT attached
Molek overlapping MolekFamily 
----> MolekFamily set attached to true

This confuses me a little. Where would it go?

What do you mean? Where what would go?

What do you mean? Where what would go?

I meant to say, where would the script go in terms of lines? Also, would I have to replace every instance of molek with the family in the script?

That's why we use families:

> Molek is attached
MolekFamily is NOT attached
Molek overlapping MolekFamily

I figured as much. Would I use MolekFamily instead of Molek for all the preceding code as well?

Just so I can figure out if I got the aforementioned code right:

But I must be doing something wrong, because very little has changed.

That's only a small part of the code. Did you implement all other steps? It feels like you don't fully understand the idea.

Here is a demo - click any molek sprite to destroy it and break apart the remaining chain.

Very cool. I see what you mean now. But now it leads to another question: with everything implemented before (That is here: construct.net/en/forum/construct-3/how-do-i-8/attach-sprite-specific-way-183986), with everything formerly worked on there, how would I now implement the new into the old?

with everything formerly worked on there, how would I now implement the new into the old?

It's your game and only you can answer this question. I mentioned before that your system of attaching sprites with dx/dy instance variables and a dictionary was overly complex and error-prone. I understand you based it on R0J0hound's advice - he's a brilliant programmer, but he works in Construct 2, where hierarchies don't exist. In Construct 3, the hierarchy feature would be a much simpler and more efficient solution for your task. That said, you don't necessarily have to switch to it.