Míriel knows that she herself had no choice - because of her weariness, her spirit had to flee her body. (Finwë, being happy and content himself, didn't get this). No one understood how profound the weariness was.
But, elves aren't mortal - that wasn't a "permanent" choice. Once she did that, a sojourn in the Halls of Mandos should have healed her...and she could return.
In that case, it would have been Finwë's job to wait for her.
But he's impatient - after not too long, he basically petitions the Valar to demand she come back...and then the debate in question happens.
She is given a choice...tell your husband that you (may) return someday, and for him to just cool it....or tell your husband "I will never return," thus freeing him to take a new wife. Since she hadn't been in the Halls of Mandos all that long yet, she immediately said "I'm not coming back - ever!"
That is what she means by abandonment and rejection. It wasn't the choice to leave...(which, as you say, wasn't much of a choice)...it was the choice to not return, which was something she could have done. Tolkien implies that she didn't really think about what it would mean to stay disembodied for all eternity when she made that choice.
I agree that Finwë doesn't come across as being that understanding...but he has no basis for relating to what she's gone through. He is happy to be a father, and doesn't understand why it's a burden for her to be a mother. Compound that with the fact that elves don't get sick, so there is really no...language...to describe what she is going through. All she can say is "I'm soooo tired." And all he can think is, "well, get some sleep, then!" not realizing that by tired, she means her soul is withered.
Given enough time to come to terms with her death, he may have grown to be more understanding. But then he met Indis

Besides playing with the idea of a "second love," I think he was working out the practicality of marriage among immortals. His own understanding of marriage was that it was "lifelong" - til death do us part. But, what does this mean for beings whose lives last as long as the earth? The simple answer is "they can marry once, and that's it." But he wanted to figure out what constraints would "dissolve" an immortal marriage. The constraint put on Míriel - never to return to her body - is the prerequisite for dissolving an Elvish marriage. You don't just have to die - you have to promise to stay dead. Slightly macabre, perhaps, but that's why he has the Valar debate all sides of the issue, so that the best for Míriel, Indis, and Finwë can be considered. They do agree that Finwë, while 'wanting' more children, doesn't exactly have a 'right' to them...the universe doesn't owe him a big family
