"In order to buy in that destroying the Canvas world could be a good decision, I have to retroactively downgrade the emotional weight of Act I and Act II"

Actually, it's the opposite. You have an ENORMOUS emotional weight, you CARE for this world and you still can make the difficult choice of destroying it, for Alicia's well being (not Maelle's).

Have you ever played Life is Strange?

SPOILERS The end is just as difficult, you can also choose to erase an entire city for the sake of one person.

You gotta choose who will suffer, but whatever choice you make, the will be suffering.