Too early to take decision that the HC12s are burnt out.
I hope that you made the setup as per Fig-1 of Post-5 and you have also loaded the respective sketches.
Now, check the continuity of the jumpers one-by-by using the following setup (Fig-1). Take out all the jumpers from UNO-1 and HC12-1. Check the continuity of a jumper and then put it back. Do the same for all the jumpers of both UNO-1 and UNO-2.
Figure-1:
If the final conclusion is that the HC12s are fried, then I would collect another set of HC12 and make them work. We have to be sure that the present HC12s are really fried.