I could reply by "I'll never cease to be amazed by how great a job Mélenchon's opponents have convinced their voters that he's really anti-EU" and link to an irrelevant picture but I will choose not to do that and engage in a real conversation. I have already explained above what his stance is, quoting his exact words and not some hypothetical hidden agenda he has, I would like to know what makes you so convinced that he is anti european then. If your opinion is that he detests the current state of Europe, then that is not a secret for anyone but there is a chasm between that and being against Europe. A fair & equal Europe is the dream of the vast majority of the people on the left.
On the subject of negotiation, I think it's fair to disagree with his strategy but in a world where the main factor in negotiation seems to be the balance of power (balance of power between companies and employees in les "plans sociaux", between strong nations and weaker ones - see Greece vs the EU, Russia vs Ukraine, etc.) and not the long term benefit of the parties involved, I understand what the reasoning behind is and I think it is sound. Of course, a quote of Camus always comes to mind in situations like these: "Good intentions can do as much harm as malevolence if they lack understanding" but I think the understanding is here, and honestly between this and the alternative of a future implosion that I am quite convinced will happen, I think it's worth it to at least give it a try for the good of the EU.
On the subject of terrorism, and the migrant crisis which are two different topics that are related to the instability in the middle east, I think we have to not be shortsighted and always remember the reasons that led to this situation. The main reason being the ill faithed American war on iraq which sadly quite a few of the EU countries backed, but not France thankfully. In parallel, the financial and economic crisis in the different EU states did not materialize from nothing, it is also a direct consequence of the economic system in these countries and there are no guarantees that a new crisis won't hit the western countries again in the next few years since there hasn't been a real change in how western economies are run since. It's still the same recipes, and if that ever happens what do you think the result will be? On both subjects, Mélenchon has been right in the past, he was a staunch opponent of wars and he has always campained for a less capitalist Europe and that is why I feel like he deserves to be trusted. I'm surprised to read you say he doesn't want further integration, on the contrary that is exactly what he wants, not a union where individual economies are competing against each other to the death. On the left, I can point to a few intellectuals, politicians and economists that call for the elimination of this EU to rebuild a completely different one based on cultural & social exchange first, Frédéric Lordon comes to mind for example, but Mélenchon is not exactly one of them. I do agree that he is not afraid to contemplate the elimination of this Europe in favor of a different one while holding on to his ideals of peace.
He is a big believer in referendums. His programme if elected is that whatever plan he negotiates will be voted on by the french people and if the result is not in his favor he will resign.