Zagreus The Heir to Zeus's throne?

From what I've read, Zagreus was the son of Zeus and Zeus's Daughter Persephone (wow, wtf?) of which he disguised himself as a serpant and "took advantage of her." She gave birth to the Godling that Zeus was willing to give the throne to...wtf? Yet years earlier he learned in a Prophesy that the 2nd child of his wife Metis would overthrow him, so he ate her while she was pregnant with Athena (thus Athena busting out of his head). I'm confused as to why Zeus who was married to Hera, would have like 5 affairs with other women (Mortals, Gods, and Daughters) and not just pick a child to make the heir that he had made with Hera. Zagreus Apparently was secretly ordered to be ripped apart by the remaining Titans by Hera. I'm also sorta fuzzy with the many versions of Dionysus being the 2nd Zagreus because Semele was impregnated with the "still beating heart of Zagreus" to make Dionysus. I'm also concerned in Hera's Jealousy issues where she continuously tries to Kill Dionysus after she took his 1st life as Zagreus. HELP CLERIFY ISSUES PLEASE!