Why does the Torah need to insert the (otherwise superfluous) word "ve'Chol Ziknei ha'Ir"?
Sifri: Because we would otherwise have thought that only five Dayanim are required, similar to the five Dayanim from the Sanhedrin who did the measuring - above in Pasuk 2. 1
See Torah Temimah, note 46.
Why do only the Zekeinim wash their hands and declare "Yadeinu Lo Shafchu es ha'Dam ha'Zeh", and not the whole town?
Oznayim la'Torah: Because then rest assured that the murderer would join them, in order to secure a Kaparah for his sin - and even if he were to admit to the murder, he would not be believed, based on the principle 'Ein Adam Meisim Atzmo Rasha'. 1
A person is not believed to make himself a Rasha by virtue of his testimony - because a person is not believed to testify for or against a relative, and 'every person is relativ to himself'.
Why does the Torah insert the 'Hey' in "ha'Eglah" and in "ha'Arufah"?
Sotah, 46b: The 'Hey' in "ha'Arufah" teaches us that the elders must wash their hands in the same location where the Arifah takes place. 1
Chulin, 11a: The 'Hey' in "ha'Eglah" teaches us that, when the elders wash their hands over the calf, it must be complete. 2
Chulin, 24a: The 'Hey' in "ha'Arufah" comes to disqualify a Parah Adumah that one killed by way of Arifah. 3
What is the significance of the elders washing their hands over the Eglah Arufah?
Chizkuni and Riva (in Pasuk 8): It signifies that just as the Zekenim's hands are clean, so too are they clean (innocent) of the death of the victim (and are not to blame that the murderer has not been judged 1 - RS"R Hirsh).
Why does the Torah add the (otherwise superfluous) word "ba'Nachal"?