I would firs just wait a little bit, there will be answers and explanation...
and last but not least, did someone of you read all the disclaimer etc. of SteamWorkshop...? Maybe you give them the full right to use and modify your Mod, as soon as you upload it to Steam? (I don't know if this is the case, but it wouldn't surprise me... for e.g., every picture you upload to Facebook, Facebook got the full rights to use and do with this picture (even selling!), whatever they want... just think about it!)
Yep Steam get‘s the right to do so.
But not UG - which would be necessary.