After unlocking my bootloader again I was able to get into recovery. From there it was easy going to install the Guru.zip via sideload ("Install zip" didnt work for some reason, said something similiar to "Couldn't mount /sdcard/". But sideload worked fine.). Updated everything and my phone runs now like it always had!
Again: really, really thanks a bunch! Thought I somehow ruined my phone, but thanks to you all I got it done. I really appreciate your effort in helping me! You guys are awesome.
PS: I won't do anything stupid with my phone again, learnd this the hard way.