Hi,
as we currently can load only one mod at a time is it possible to merge/edit existing mods (wich touch very different things) with the Dev Kit? I’m very new to UE4 and failed trying exactly that.
Some hints will be much appreciated 
yes, but from what i can tell, only with the mod creators cooperation.
If you download a cooked mod, the DevKit will not be able to edit it. You would have to get the cooperation of the creator of the mod you would like to merge, and get the uncooked files of his mod. From that point you should be able to merge the mods into one file fairly easily.
I dunno about “fairly easily”. When 2 mods modify the same class, how do you resolve? There are different approaches, and none of them are easy, and all of them are labour intessive hehe.
There are some discussions of a mod loader but no one has taken on that project as far as I understand. It would require a lot more knowledge how ue4 operates for me to even know how difficult it would be.
Ultimately how you resolve an asset conflict (outside of test game mode or game data which must be merged) you would just give priority to one mod or the other. Depending on what the mods do the asset could be merged as well but it all depends.
Like I said… labour intensive. Then you get into the whole philosophical debate of who is deciding who’s assets get priority. Good luck with that!
Not really a philosophical debate, the user would just order them in a list. Works the same way for Arma 2 and 3 (selected by user) and Minecraft (selected by mod developer).