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Assuming חופה is דרבנן one could answer the questions about the נוסח of ברכת אירוסין this way.
Since חופה is דרבנן we need to make the ברכה by קידושין, which doesn't work on the עריות, and in order to make sure no one mistakenly thinks that someone can sleep with the girl they are betrothed to, we need to clarify that ״שאסר לנו את הארוסות״... and that it was only מותר through חופה and קידושין.