Weinstube, DO NOT ERASE STEAM, you don't need to. I use to have the same problem

riginally bought SH5 from steam, where I play ARMA 3, and I run it for a wile, but only to be "forced" to get another copy from a different source,just to be able to fully modding it.Which seam to me the same situation of yours.Like I told you, JUST CREATE A NEW U-PLAY ACCOUNT WITH A DIFFERENT USER NAME AND E MAIL.
I first whent on the steam library, right clicked on sh5 and then "erase all file". This erased sh5 from steam folder. Then, just to be sure, I went on control panel/programs and func. and uninstalled sh5.exe.Erased also the sh5 folder in C:/documents.Then installed new Sh5, patched to1.02 and then registred on U-play at the first run with a new user name and e mail. That will link the new sh5.exe to u-play WITHOUT steam.
Hope it help!