To answer your question, Job 1:6 says they were with Satan. Who else got to challenge the Almighty God? They were not some human authority figure who went to threaten or tempt Job, they came to God, in His presence, in His heavenly court, even though they were "roaming on the earth". Who else could they be other than demons?
Then you go to Ps. 82:6-8, judgement was declared, they will die LIKE MORTAL MEN. Now that obviously indicate that their immortality was stripped away, they were degraded from somewhere superior to a mortal level, and their fate was sealed. If they were mortal men to began with, why would that be a punishment? Wouldn't they die in one way or another? In this passage, God was crashing their ego. These sons of God thought they had the privelege, they were elohim/deities, they thought they could challenge God, but ironically, soon they would die just like those mortal men whom they looked down upon.
I'll address this issue more shortly. However, everything you've said here is an "inference". My friend, I again encourage you to put away the NLT. I looked at the Greek and the Hebrew and neither say, "LIKE MORTAL MEN". They both say "Like men". This is a good example of translator bias. The translator(s) thinks this passage is about spiritual beings so he/they adds the word "mortal". By doing this he/they cooks his/their own bias into the translation and misleads the reader. There's nothing in the passage that says they are spiritual beings. If we look at the context, we find the Psalm asking, how long will they judge unjustly and accept the wicked? They are told to defend the poor and the fatherless. Are we to suppose that the demons will judge justly? Are we to suppose that demons will defend the poor and fatherless? Are we to suppose that the demons are going to do justice to the afflicted and needy? Are we to suppose the demons are going to deliver the poor and needy? Surely, these are men.
At the end of the Psalm we see a call for God to judge the earth, for he will inherit the nations. If this passage is talking about demons why is God called to judge the nations. Shouldn't the call have been to judge the angels of the demons? We don't have to guess. If we read on we see who these enemies of God are. He says the enemies of God have set themselves against His people. Then he goes on to list the nations that he's talking about.
"The tabernacles of Edom, and the Ishmaelites;
Of Moab, and the Hagarenes;
7 Gebal, and Ammon, and Amalek;
The Philistines with the inhabitants of Tyre;
8 tAssur also is joined with them:"
The context of this whole passage is about men. In verse 6 he says, you are gods and the children of the Most High. In context it's clear that these sons of God are men.
1 God standeth in the congregation of the mighty;
He judgeth among the gods.
2 How long will ye judge unjustly,
And gaccept the persons of the wicked? Selah.
3 Defend the poor and fatherless:
Do justice to the afflicted and needy.
4 Deliver the poor and needy:
Rid them out of the hand of the wicked.
5 They know not, neither will they understand;
They walk on in darkness:
All the foundations of the earth are out of course.
6 I have said, Ye are gods;
And all of you are children of the most High.
7 But ye shall die like men,
And fall like one of the princes.
8 Arise, O God, judge the earth:
For thou shalt inherit all nations.
Psalm 83
A Song or Psalm of Asaph.
1 Keep not thou silence, O God:
Hold not thy peace, and be not still, O God.
2 For, lo, thine enemies make a tumult:
And they that hate thee have lifted up the head.
3 They have taken crafty counsel against thy people,
And consulted against thy hidden ones.
4 They have said, Come, and let us cut them off from being a nation;
That the name of Israel may be no more in remembrance.
5 For they have consulted together with one consent:
They are confederate against thee:
6 The tabernacles of Edom, and the Ishmaelites;
Of Moab, and the Hagarenes;
7 sGebal, and Ammon, and Amalek;
The Philistines with the inhabitants of Tyre;
8 tAssur also is joined with them:
They have holpen the children of Lot. Selah.
9 Do unto them as unto the Midianites;
As to Sisera, as to Jabin, at the brook of Kison:
10 Which perished at En-dor:
They became as dung for the earth.
11 Make their nobles like Oreb, and like Zeeb:
Yea, all their princes as Zebah, and as Zalmunna:
12 Who said, Let us take to ourselves
The houses of God in possession.
13 O my God, make them like a wheel;
As the stubble pbefore the wind.
14 As the fire burneth a wood,
And as the flame setteth the mountains on fire;
15 So persecute them with thy tempest,
And make them afraid with thy pstorm.
16 Fill their faces with shame;
That they may seek thy name, O LORD.
17 Let them be confounded and troubled for ever;
Yea, let them be put to shame, and perish:
18 That men may know that thou, whose name alone is JEHOVAH,
Art the most high over all the earth.
The Holy Bible: King James Version, Electronic Edition of the 1900 Authorized Version. (Bellingham, WA: Logos Research Systems, Inc., 2009), Ps 82:1–83:18.