You don't have to remove Melodyne, just run the updater program and deactivate that PC.
If you cannot do this, just email them and they will remove it for you (it took like 10 mins for me).
You can run Mel Editor on 2 PCs anyway.( I have Editor also).
I think the CD key is the one you want. An OEM license should work since this is what I used.
If you have already used this license to update the PC to W10 (and rolled bacK), this will work too.
If you used this license to update another PC, it will not work for both, at least in the long run.
One license for each PC.