Why did Hashem incorporate Aharon in the Dibur?
Ramban: Because, following the report of the spies, Aharon fell on his face together with Moshe. 1
Ramban: Nevertheless, Hashem continued to speak to Moshe alone - in Pasuk 28, since he was the more prominent of the two when it came to prophecy.
Which 'Eidah' is Hashem referring to here?
What does the latter half of the Pasuk then mean?
Seforno: Hashem is saying that, despite the fact that it was the Meraglim who caused the people to complain, He heard their complaints, and was ready to implement them.
And why did Hashem not forgive the spies?
Seforno: Because they were guilty of sinning and causing others to sin, as the Torah explains "asher Heimah Malinim alai".