What is "Lev Shome'a"?
Radak citing Targum Yonasan: A heart that understands. And so he translated "Lehavin Bein Tov l'Ra."
Why did he call them "Amcha ha'Kaved"?
Rashi #1: Because they are many, they have many dealings, and they come for judgment. I do not have time to investigate their law.
Rashi (from Sifri Devarim 9): Their judgment is very weighty. If I will take from one and give to the other improperly, I mortally punished 1 him - "v'Kava Es Kov'eihem Nafesh" (Mishlei 22:23).
Radak citing Targum Yonasan: It is numerous. The same applies to "Mikneh Kaved Me'od" (Shemos 12:38).
Another version of Rashi: If two Nochrim come to me for Din, and I take from one and give to the other improperly, I am not punished. If they are Yisraelim, I am mortally punished.