What is the significance of the fact that Moshe spoke to Yisrael in the fortieth year?
Seforno: He spoke to the new generation who were about to enter Eretz Cana'an, after the old generation, who left Egypt, had all died.
What is "ke'Chol asher Tzivah Hashem oso ... " referring to?
Seforno: It teaches us that Moshe reviewed the entire Torah, all that he had been taught until now.
Why did Moshe wait to rebuke Yisrael until now?
Rashi: He took his cue from Ya'akov, who rebuked his sons only shortly before he died. 'Reuven, my son' he said to his B'chor, 'I did not rebuke you all these years 1 so that you should not leave me and join Eisav my brother'. In fact, the Sifri cites four reasons for only rebuking close to one's death: 1. In order to avoid having to rebuke many times; 2. In order that the one who has been rebuked should not be embarrassed upon meeting his rebuker; 2 3. In order that he should not bear him a grudge and 4. In order that he should not leave him and go elsewhere.
In verse 1, it says "El Kol Yisrael." Why does it say here "El Bnei Yisrael", and afterwards (verse 5) Stam "Ho'il Moshe Be'er Es ha'Torah"?
Moshav Zekenim (1, citing R. Yehudah ha'Chasid): First, Moshe said Aseres ha'Dibros to everyone. Then he told the severe [Mitzvos] to part of Yisrael and the Chachamim. Afterwards, he said Stam the explanations and the  names [of Hashem] that come from the verses Shemos 14:19-21 (see Rashi Sotah 45a DH Ani).
QUESTIONS ON RASHI
Rashi writes that one should rebuke close to death, lest he need to rebuke many times. It is a Mitzvah to rebuke even 100 times!
Riva #1, Moshav Zekenim: That is when he is about to sin, but for what he already sinned, one rebukes only close to death.
Riva #2: The concern is lest Reuven rebuke Shimon for an Aveirah, and afterwards Shimon will see Reuven transgress it. He will say 'fix yourself before you fix others!', and will despise the rebuke he received. Therefore, one waits until close to death. 1
Riva: However, 'Mochicho v'Chozer u'Mochicho' implies unlike this.