Is "Negid Tzor" the king?
Radak: Yes.
Malbim citing Mahari: No. Below (12) discusses the king! Rather, it is someone appointed over matters of the country.
How was his heart elevated?
Malbim: This is thinking that he is at a higher level than he truly is. It is an abomination to Hashem ? "To'avas Hashem Kol Gevah Lev" (Mishlei 16:5).
Does he consider himself Elokim?
Radak: I am like Kel ? no one can harm me (refer to 28:2:4:2). All merchants of the nations serve me and bring to me gifts.
Radak citing Targum Yonasan: I am feared.
What is the significance of "Moshav Elokim"?
Rashi: He made a dwelling in the air on the sea, like seven Reki'im, and dwelled on the top one.
Radak: I sat in a strong place, b'Lev Yamim. Seas surround me. Who can touch me?! This is like Elokim, who dwells in Shamayim [and no one can harm him].
Radak citing Targum Yonasan: I have a strong dwelling in the sea.
Malbim: He dwelled apart from all people, in the heart of the sea, just like Elokim dwells away from people.
What do we learn from "v'Atah Adam v'Lo Kel"?
Malbim: There were people who had Elokus in them due to their Nefesh, e.g. "Moshe Ish ha'Elokim" (Yehoshua 14:6), and Daniel had "Ru'ach Elakin Bei" (Da'as Noteh Kri'as Shma 6:4). However, you are just a man, and not Kel. There is no Elokus in you, just you put your heart like Lev Elokim!