What are the Se'irim to which the Torah is referring?
Targum Yonasan: They are gods in the shape of demons.
As in Yeshayah, 13:21. They are so-called because they live in desolate places ('be'Makom Shadud' [Ramban]).
Which many people worshipped like gods (Rashbam in Pasuk 5. Ramban: And as Yisrael indeed did in Egypt. Refer also to 17:7:3:1*****.
What are Sheidim?
Ramban (citing Chagigah, 16a) and Seforno (in short): Just as Hashem created all physical beings from a combination of the four elements (fire, air, water and earth), 1 so too did He create Sheidim, spiritual beings out of fire and air, 2 that are invisible, that possess wings and that are able to fly 3 (like angels), but that eat and drink, multiply and die (like human-beings). 4 They are also able to foretell things 5 regarding the immediate future 6 which they pick up from the masters of the Mazalos as they fly high in the sky. 7
Ramban: All of which can be 'felt' with the five senses.
Which cannot be 'felt' due to the lack of physicality of these two elements (Ramban). Sheidim are also known as 'Mazikin, which, according to one opinion in Avos (5:6) were created at dusk of Erev Shabbos.
Like birds, whose chief components are fire and air.
See also Ramban DH 'u'le'Kach' and Seforno with regard to what they eat.
Which explains the current prohibition - even though they did not accept them as deities (Seforno) Refer also to 17:7:1:1*.
See Ramban DH 've'Eilu ve'Eilu'.
Similar to birds (See Ramban, Devarim 15:9, DH 'Lo Silmad La'asos').
Why does the Torah go to such lengths to prevent the people from having contact with the demons?
Ramban: Because slaughtering to the Sheidim is an (attractive) form of idolatry. 1
Seforno: Because, since their subsist on vaporized blood, the people may slaughter animals to share a meal of blood with them, in order to befriend them and gain their assistance to indulge in the pleasures of life that are not connected with the service of Hashem. 2
What s Chukas Olam Tih'yeh Zos lachem le'Doroseichem" referring to?
Oznayim la'Torah: It refers to the Isur of Shechting Kodshim outside the Azarah. 1
See Oznayim la'Torah.
"V'Lo Yizbechu Od? la'Se'irim" implies that now, Yisrael do offer to Shedim!
Ramban: They used to do so in Mitzrayim.
Why does it say "Zonim Achareihem"?
Moshav Zekenim: Seeking demons and trusting in them is like adultery [Yisrael are considered Hashem's wife], for he thinks that someone can help or harm him, other than Hashem.
QUESTIONS ON RASHI
Rashi writes that "Se'irim" are Sheidim. Why does the Torah refer to them as "Se'irim"?
Ramban #1 (citing Ibn Ezra): Because their appearance is so frightening, that the hair (Se'ar) of whoever sees them stands in end.
Ramban #2 (citing Ibn Ezra):: Because crazy people see them in the form of goats.
