Paragraph 2: Don’t know what this means.
It's what they discuss, at times explicitly, about the difference between memory/art and the real thing.
There is the real Heloise and there is what Mariane sees. The portrait that Mariane creates is a reflection of how she sees Heloise. As Mariane's feelings about Heloise change, so does the portrait. The two of them have several conversations about what it means for someone to create a representation of you.
Paragraph 3: They could have stayed together. The dark-haired one said she would never marry & she was going to inherit her father’s business so there would have been financial means. The blonde lady would have had to defy her mother & the arranged marriage, but it was doable. I mean, when did lesbian love begin? As far back as anyone can remember?
They could have perhaps stayed together, but it's not ultimately what either character is willing to go for. And their life together in reality would not be the life in which they fell in love (nice house, maid, no outside social disapproval, etc).
In the film, the characters discuss the myth of Orpheus. Mariane explicitly says that maybe Orpheus gives up Eurydice because "He chooses the memory of her. That's why he turns. He doesn't make the lover's choice, but the poet's." Mariane is articulating that sometimes a memory or a representation of someone is preferable. As a memory, Mariane can love Heloise easily. In fact, Heloise says "Maybe she was the one who said 'turn around'". In other words, sometimes an object of affection might prefer to exist as a memory. Heloise and Mariane exist in this temporary utopia, but they know it can't last (as symbolized by the flowers that slowly die in the kitchen).
They do explicitly discuss fighting against the marriage, but they say this:
Heloise: You would prefer me to resist?
Mariane: Yes.
Heloise: Are you asking me to? Answer me.
Mariane: No.
Then there's the moment where it is Heloise who says "Turn around" (going back to the myth) and Mariane sees her in the wedding dress. Heloise is choosing to be a memory.
They choose to give up their relationship. For Mariane, it seems to come from a place of making the "poet's choice"--to have wonderful memories of someone instead of having to deal with the messier reality of loving a real person.
And for Heloise, it seems to come from a place of not wanting to fight against pressures. To continue to enjoy material wealth, be on good terms with her family, and to be a lovely memory in Mariane's mind.
Again, I really vibed with this movie. It was slower in its pace, but I thought that both the visuals and the themes were interesting and well-executed.