#(which kinda sorta implies that Zeus was *maybe* courting or even married to Leto at that time)

[my-name-is-apollo]

Why is Hera so hostile to Leto in a manner that she isn't with the other lovers of Zeus? I can't think of any other woman who was targeted so much by Hera. One could say she didn't want Leto to give birth because her children would be a competition to Hera's children, but why bother her even after she had already given birth? Is it because Leto herself is also a threat to Hera?

Sort of I guess?

I don't think Leto would ever be a threat to Hera's role as the queen of the gods but Hera might see her as a competitor for Zeus' affection, even though I think Zeus would not marry someone else and replace Hera.

The twins are definitely a big reason why Hera begrudged Leto - it is outright stated in the Callimachus Hymn to Delos that the reason Hera especially targeted Leto was because she was told that Apollo would be dearer to Zeus than Ares is. Zeus is very proud of Artemis as well. As he himself puts it, he doesn't mind facing Hera's wrath for children like her.

But the continued hatred even after the birth of the twins (like sending Tityus to rape Leto) could have been for different reasons. This wasn't like one of those affairs Zeus would have with mortal women where he'd leave them behind once the child is conceived. Neither could Hera, despite her many attempts, get rid of Leto like she did with the other lovers. Not only did Leto give Zeus children that he loves dearly, she also stayed on Olympus despite Hera's hatred towards her (which isn't directed to any of Zeus' other divine mistresses, btw). It might have also been because Leto herself is dear to Zeus, if the way she's treated on Olympus is any proof. In the Homeric hymn to Apollo, Leto stands next to Zeus - in the Olympian assembly - to welcome their son. Hera is completely absent from the scene, as if Leto had taken her place even if temporarily. Now you could say this is because Apollo himself is such a powerful and glorious son in a way that no other son of Zeus is, so of course Leto would get such an honor (the hymn itself presents it this way).

But here's another instance - in the Iliad, when Hera goes to seduce Zeus, he is obviously very smitten but before getting into the action, he lists some of his lovers (I believe these were his favorite lovers, as a lot of others are not mentioned):

"for never has such desire for goddess or mortal woman so gripped and overwhelmed my heart, not even when I was seized by love for Ixion’s wife, who gave birth to Peirithous the gods’ rival in wisdom; or for Acrisius’ daughter, slim-ankled Danaë, who bore Perseus, greatest of warriors; or for the far-famed daughter of Phoenix, who gave me Minos and godlike Rhadamanthus; or for Semele mother of Dionysus, who brings men joy; or for Alcmene at Thebes, whose son was lion-hearted Heracles; or for Demeter of the lovely tresses; or for glorious Leto; or even for you yourself, as this love and sweet desire for you grips me now.’ (Book 14, trans. A. T. Murray)

Notice how when talking about most of them, he also mentions the children they bore to him but when Demeter and Leto are mentioned, he doesn't bring up their children at all despite them being some of the most accomplished kids of his. What's more, he takes Leto's name just before Hera's. I mean, this is an interpretation but it looks like not only did Zeus love Leto the most out of all his mistresses - giving her a place second to that of his wife, but also his love for her wasn't necessarily only because she gave him two amazing children.

Nonnus does something similar in the Dionysiaca (but this time Zeus is enamored with Persephone instead of Hera) but more notably, when Typhoeus attacks Olympus and Zeus is discouraged, Nike takes the form of Leto to encourage him and it's pretty telling of what Leto meant to Zeus.

One interesting similarity between Hera and Leto is that they both had a giant try to rape them. Porphyrion tried to violate Hera (Zeus inspired him to do this) and Tityus tried to violate Leto (upon Hera's order). Though both of them were killed, only Tityus got an eternal punishment in Tartarus of having his liver/heart eaten out by vultures so Zeus seems to have taken a greater offense at Tityus trying to assault Leto.

Again, I don't think Zeus would ever take anyone other than Hera as his permanent wife - she is irreplaceable to him. There's an entire myth about Hera leaving him and Zeus winning her back. Their relationship is obviously complex and involves all kinds of emotions including love and hate. But Leto is continually dear to him as well and that's something Hera can't do much about.