Non-Freeform drag and drop interaction help

I am really puzzled by this interaction I am trying to build. I may possibly be trying to do too much. I was hoping for some advice on whether or not what I would like to do is possible. If it is, some insight on how to accomplish it would be greatly appreciated.

I am building a drag and drop interaction where I would like to be able to eventually combine any two measurements to see how you would convert them from one measurement to another. The way it is currently set up, it directs the learner to first select the FROM measurement unit and then only provides a selection of measurements to convert TO that were chosen by the instructor. I would like each measurement unit to be able to be dropped in either drop zone, this is why I am not using a freeform drag and drop interaction. I have attached the slide I put together to attempt this.

Here are the issues I am having:

1. I am not able to see a way to only allow one measurement per drop zone. Is there a way to accomplish this using variables and triggers? (It would be nice if the contextual right-click menu of the drop object or zone included the option to only allow one item. Currently, it only shows stacking options.)
2. In this example, I have disabled the TO field because the instructor had a selection of equations they wanted to provide. I change the state to Normal when the FROM drop zone is populated (the state is something other than Normal). Even with the TO drag area disabled, I can still drop objects into it. This also happens when the object is hidden.

I also wondered if it would be possible to clear one or both drop zones without having to reset the slide to its initial state.

I would love to hear any suggestions about how to accomplish this.

Thank you so much for your help!