I have a feeling that the heroes are reincarnations too (knight's soul). If you think about,it the demon lord's soul is made of 2 souls the original demon lord and that of the knight. I also think that the goddess pitied the fate of Jun and the knight so that she made it so part of the knight's soul is reincarnated as well together with Jun's (kind of like deal, hence not a permanent solution and where it justifies the part of what Graham was taking with the Goddess). Also the part of the soul of the knight that gets reincarnated does not have any memories of its past life, while the other part still remains with or as the demon lord. Another point is that the reason that Jun keeps on reincarnating is that she still hasn't accomplished the reason she was summoned, she still hasn't turned the demon Lord's soul back to a human one.
Well all but the last one lol, but this theory seems pretty good.
So what I'm getting from this is that every time she falls in love, she grants that person the abilities of the hero? In other words, the heroes aren't actually pure and righteous people selected by the goddess, and it's not that all heroes suck like she thought, but she just has bad taste in men?