'Jackie' is just Moon shapeshifted to look like a teenage girl.
Have we ever seen the two of them together?
Either the memories people have of a young Jackie are false, or there is a real Jackie locked up in some Mewni dungeon.
Moon thinks Star may not be capable of being queen. Therefore, he is pursuing Marco. If Moon is succesful and is impregnated by Marco, she will bear a new heir more suitable for the throne, with Marco's safe and organized genetics rather than River's foolhardy act first think never approach. On the other hand, if Star wins Marco back from her, she will have proven herself to be a capable young woman worthy of ruling the throne alongside Marco. Either way, Moon wins.