No access to launch codes, and no access to required maintenance skills. Even the US said shipping them to the US would be pointless, too, because the US didn’t have that either.
They never had those nukes. Russia still had them, even though on Ukraine soil.
The whole “if we had not given them back” stuff is just wrong. They never had them.
Actually if they still had them & Russia targeted them, Russia would still have to trigger the fission process. Another bomb would not even trigger it, from my understanding. I suppose there might be some kid of way, but I am not familiar with it. In other words, I have never read anything that say how that might be accomplished.
You are correct, Ukraine giving them back took nothing away from Ukraine's defense whatsoever.