question

I doubt changing the slug building mode during the runtime is properly supported. It might be fine if it's done while the conveyor is empty. But turning the slug building off while there is a slug waiting will very likely always break the logic.

You might have more success by implementing your own custom slug building logic (Process Flow is probably easiest). You can use a decision point or photo eye as the point where the items wait, and stop arriving items depending on the current mode. Then use the "On Item Bump" event in Wait for Event or Event-Triggered Sources to collect a slug.