As we finish the מאמר, the רבי uncovers the depth within "נעשה ונשמע", and the preparations necessary for מתן תורה:
Why indeed must "נעשה" come before "נשמע"?
The answer is, "נעשה" is true ביטול, and "נשמע" is fulfilling what Hashem wants. In order for the ביטול of "נשמע" to be complete, it must be preceded by "נעשה", and in order for the ביטול of "נעשה" to be aligned with Hashem's goal, it must be followed by "נשמע"!
With this, we understand why we learn out the preparations for מתן תורה, not from "מדבר" - total ביטול - as would be intuitively perceived (and as תוספות suggests in his question), but from "היום הזה" - expression of self: Once we align our identity of self as total ביטול, we then fulfill our purpose of having been created, by expressing this 'self' and making a דירה בתחתונים!