I think we must clarify some minor things. The word "god" is not different in any way than the word "demon" or "dragon" or "wizard" when it comes to FTverse. I say this because we've see God Slayer Magic, Demon Slayer and Dragon Slayer. These magics, from my point of view, differ in power, yes, but more in terms of intention. I don't see much difference in Gray's Ice magic. Its still ice, be it Ice Magic, Ice Make or Demon Slayer Ice Magic. Its Ice. However, after his dad's demise, he became able to infuse it with the will to harm demons. Same thing, I assume, happens with the other slaying magics. Now, why do I put god in there? In FTverse, the strongest mages in Ishgar were called "gods". We saw a god slayer. Yet his magic was incapable of slaying a dragon or a human like Makarov. It was stronger but it was more than its nature was different than actually its power. We understand that magic in FTverse is direved from feelings, emotions, intents...will power. Conjuring fire is simply that. But embuing that fire with your will can change its nature and allow it to affect different things. Also, just food for thought but... The king of the celestial spirits is, to all intents and purposes, a god. Its simply a matter of perspective all around. We are locked in this notion that the word god inherently brings about a being that is able to control existance. However, in FTverse, it seems more like a "power" adjective given to some very powerful people.
Why do I bring this about? Simple. Anhksheram can be anything from an abstract concept (like the example Caliburn gave with Fukushima and other natural disasters) like fate, to a powerful wizard from the past to even the first being to have magic. This said, I doubt that Zeref's curse is directly a result of a god punishing him from above.
Look at how the curse manifested itself. Zeref was stricken by the reality that he cannot bring back his brother. In that moment of anguish and sorrow, his feelings awaken something inside him. And the "curse" manifested. This seems more in tune with a dormant power he possessed that awakened in that moment. It wasn't like the situation with Mavis. She used an incomplete magic which inherently injured herself despite bringing about the required result. Zerefs different because he was simply distressed at the time when it first showed. He didn't perform and wasn't performing any magic when it happened. If he had it or acquired it afterwards, is unknown to me.
I mean, a Curse power like the ones that the demons use is nothing more than a type of Magic. Magic is simply using ether or ethernano or wtv you want to call it, both the one inside you and the one in nature. Using it to create these supernatural feats. Demons are creations from ether itself that use ether. Curses are nothing more than forms of magic. But magic none the less. Zeref's curse is nothing more than a magic power and therefore more likely to be inherent to him and not a result of some external factor.
FH I feel will be shown as a magic spell or power capable of changing the world. I feel it will be something of the scale of time manipulation or reality warping.
I agree with you cali, though. Dark, Light, etc is all just a matter of perception. Dark Magic is in essence not different from Light Magic. Think about the fight between Jose and Makarov. Dark Magic causes pain, death, suffering. To an identical scale, so does Light Magic.
I still think Mavis' condition is different from Zeref. I really do. To which extent I'm unsure but I feel its different.