Edit: I think OP edited their message.
https://nos.nl/nieuwsuur/artikel/2213767-dutch-intelligence-...
https://arstechnica.com/tech-policy/2018/03/dnc-lone-hacker-...
As for Guccifer, the evidence indicates he operates out of CST (based on tweets and uploaded file timestamps), hardly where a Russian hacker would be situated.
These are both pretty weak as evidence, not what i'd call substantial.
Meanwhile, of the above two articles, one (the Dutch intelligence bit) has nothing to do with the DNC. This leaves only a rather shady character, Guccifer 2.0, whose actions are pretty unprecedented in history: a hacker who announces what they did, seeks interviews, and crows on social media.
Why, exactly, would Russian intelligence set up such a persona? If their intent is to aid Wikileaks, why would they undermine the Wikileaks narrative that it was a leak, not a hack? Guccifer makes no kind of sense unless we believe that his entire purpose is to point the finger back at the Russians, as he quickly succeeded in doing.