Okay, this has been on my mind the whole day. And after searching the entire internet, articles, youtube videos and what not, I cannot find a legitimate answer anywhere. So, I decided to post the question here myself. PLEASE NOTE: The question isn't why Allah didn't forgive Iblish when He forgave Adam (AS) and Eve (RA) for their sins. The Question is WILL ALLAH FORGIVE Iblish IF he decides to REPENT TO ALLAH (SWT). I came across a Dr. Zakir Nayek video during researching and while it did not answer the question (not even close) he mentioned that Allah (SWT) has filled Iblish's heart with sins (and Allah [SWT] knows the future- yes, undoubtedly). Why would He fill his heart with sins? Isn't it contradicting with the FREE WILL we Mankind and the Jinn possess? What if the Devil finally came to the realization of his horrible mistakes and decided to seek Allah's forgiveness? For indeed Allah is the most Merciful and the most Forgiving, will Allah (SWT) not forgive him? If no then why not?
Consider that this was Allah's (ﷻ) response to Pharaoh (Firown) when he "repented" after all his evil and the punishment overtook him: "Now? And you had disobeyed [Him] before and were of the corrupters" - Qur'an 10:90-91. In the end, Pharaoh's repentence was not accepted. I imagine the same would be the case for Iblis if he 'repented.'
Repenting when it is too late/inappropriate is fruitless.
Moreover, 'hypotheticals' are not real. Allah (ﷻ), the All Knowing and Seer of the Future, only declares something to be true when it is true. One example: he knew Abu Lahab would die upon disbelief, thus He declared this before he even died! (Sure people could think, "what if he repented"; but clearly he had no intention to and indeed did not! SubhanAllah).
Likewise, he declared Satan to be cursed.. which indicates Satan has no intention to repent and in fact will not repent!
"Why would He fill his heart with sins? Isn't it contradicting with the FREE WILL we Mankind and the Jinn possess?"
Whether Satan or anyone else (from humans or Jinn), Allah (ﷻ) only gives you what you choose. If you choose good, Allah (ﷻ) increases you in good. And if you choose evil, then you are increased in this as well. He says this in various places throughout the Qur'an. e.g., "In their hearts is disease, so Allah has increased their disease" - Qur'an 2:10.
Satan was envious of Adam.. to the point of refusing belief. He chose this for himself.