"This mitzva" refers to (= seems to refer to: see below) the whole Torah. However it would have been more accurate to refer to the whole Torah as, "the whole mitzva which I command you today (as in Devarim 8:1)" (and not "this mitzva").

Rather, "this mitzva" refers to repentance, that was mentioned above. The earlier statements, "You will return to your heart (30:1)," and "You will return to Hashem your G-d (30:2)," are actually commands to repent. The only reason future tense is used is to hint at the promise that this national repentance will actually take place.

The message of the parsha is that despite your dispersion being "at the edges of the Heaven" and in the hands of the nations, you can return to Hashem and fulfill all that I command you today. This is not hidden, nor far from you, but very accessible to you at all times and in all places.

This is the meaning of "in your mouth and in your heart to do it." They should verbally confess their sins and that of their ancestors and return with their hearts to Hashem, and today accept the Torah upon themselves for the generations, as mentioned, (30:2) "You and your children with all of your heart," as I explained.