Who does "Adas Yisrael" refer to?
What exactly does "v'Ne'elam Davar" mean?
Rashi: It means that the Halachah was hidden from them and they issued an incorrect ruling - to permit something that is subject to Kares.
Who subsequently sinned?
Rashi writes that Moshe knew that he will not take Bnei Yisrael into Eretz Yisrael.In Beha'aloscha (10:29), Rashi says that this was not yet decreed, and Moshe thought that he will enter!
Riva: Moshe knew that he would not lead Bnei Yisrael in, but he thought that he himself will enter.