Why did Moshe partner with the elders in this command to Yisrael?
Ramban: Having finished his words, Moshe instructed the elders to speak with people as well and warn them to keep the Mitzvos, since a nation follows its leaders. 1
Seforno: Because they were destined to be present when they crossed the Yarden.
Oznayim la'Torah: In order that they should ensure that Yisrael carry out "SHamor es Kol ha'Mitzvah" after his death. 2
Ramban: Nevertheless he said "asher Anochi Metzaveh eschem" and not 'Anachnu', since he was the main speaker, and the elders only came to back him up with 'Listen, pay attention and do!' - similar to Pasuk 9 - Refer to 27:9:1:1.
Which indeed they did, as implied in Yehoshua, 24:31.
Why is "Shamor" written with a 'Kamatz' instead of with a 'Sh'va'?
Rashi: Because it means (not 'Observe!' 1 , but) 'The onus of observing the Mitzvos is constantly upon you'.
Divrei David: Also, if it was a command, it should have written "Shimru" (plural), like the Pasuk ends "asher Anochi Metzaveh eschem ... ." In fact, Targum Yonasan translates it in the plural.
