Um..What exactly must be..? (spoiler)

I know there isn't much activity on this board, but maybe someone will drop by in a year or two that can help me understand this scene ;)

What did the movie intend "Must it be" to mean? (I know that in real life it didn't have anything to do with Johanna or the immortal beloved when Beethoven wrote "Must it be? It must be" but in the movie, it did.)

When I first saw the movie I thought it had to to with hiding their real feelings for each other. Like, "Must we keep up this charade, pretend we hate each other?" "Yes we must", says Johanna, which saddens Beethoven, but that's why he doesn't state her name in his will. (Which would otherwise be quite hard to explain.)

Then I realized that at this point, Johanna still doesn't know the story. She hasn't read the letter, she still think he abandoned her back then. There is no relationship to keep hidden, they really have hated each other all these years for what they perceived the other had done.

She must have wondered why he was suddenly so friendly with her. She was friendly, too, I suppose it was because he was dying, and because of Ode To Joy.

But what does the exchange mean? Does it have to do with Karl? Beethoven gives Karl back to his mother, but then he asks if it must be, if Karl really has to leave him? And she says yes. Is that it..?

I love this movie, but the chronology was a challenge!
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"The best fairytale is one where you believe the people" -Irvin Kershner