Don't try to do this using an action list on the UI. Instead, modify your flow to go through the steps you detail. The flow rule will take action based upon values on the clipboard, and that will be correct.
I understand your approach, we could add a fork shape in the flow to check for the condition.
But how would I open a different assignment (from a different case) then? In the Action Set I can configure "Open Assignment" and pass e.g. Primary.OtherAssignmentKey - how do I trigger this action from the flow itself?