First, I'm only quoting you point by point to organize my responses to each of the different, yet unified issues you seem to be questioning. This is not to be combative. I'm going to try to answer your questions to the best of my ability. Nothing is meant to be dismissive, antagonistic or combative in anyway. I'm new here and people don't know me so I feel the need to caveat all this.
Also, I have not read all responses and forgive me if any of this sounds like preaching to the choir.
If you think about this, we are far removed from the cultures that wrote the Bible and had no problem believing in the supernatural. Why is it that they had no problem believing in the supernatural? When you crack open the Bible, and other Ancient Near Eastern literature as comparison, they talked about these beings like it was no question they existed. They testified to their experiences with them, etc.
The point being, overall, the Bible is a book of human witness to us as well as God's revelation through that witness. In other words, human agents used to author the Word of God. General point, as I know people can get caught up in human authors versus God as author, the humans testified of what they saw and experienced.
That shouldn't really be different today if someone testifies that they experienced a supernatural entity. I have, you say you have. Other people testify they exist.
Be careful not to diminish human testimony because if you do that you diminish human testimony that has brought us the scriptures.
Ancient Near Eastern History and Ancient Middle Eastern language expert Dr. Michael Heiser argues that this is the
nachash, in Hebrew "Shining One" and not necessarily "the serpent" although it can be translated as such and elsewhere throughout the Bible, but also in Numbers 21 with the firery serpents. The point here is that these are supernatural entities. Some sources linked below for you to investigate:
Or his
Unseen Realm book.
He also argues that after the Tower of Babel during the Table of Nations God divided the nations at that time up according to the sons of God (
bene elohim) who in Job are supernatural beings (often only called angels although that is a job description) and these beings that were given supervisory roles over the nations also rebelled. Heiser goes into detail about this in his dissertation far more than I can do justice here. He shows Deuteronomy 32:8 is often mistranslated "according to the Sons of Israel" although Israel did not exist at that time. Better translations are "according to the Sons of God." See the comparisons:
https://biblehub.com/deuteronomy/32-8.htm.
Overall, these
bene elohim are and were part of the divine council God uses and used to judge nations, etc. It's not unbiblical, you see a divine council event in 1 Kings 22:19-23. Long story short, these supernatural beings rebelled and set themselves up as false gods. So not everything said to be a "god" in the OT is just a statute or demons.
You're forgetting the evil spirit that harassed Saul. David's music quelled that spirit although it did not cast it out. Just nitpicking here a little bit.
Scripture testifies of demons in the Gospels and the Epistles. Jesus has a conversation with Legion and other. He simply casts out others. Why does one have to see one to know that they exist based on the testimonies of the Apostles? As I said above, you testify to your own experience, I testify to mine, the Bible is a testimony and we have many more around the world til this day. These are eyewitness accounts of experiences with the supernatural, evil and good.
Now is a good time to answer the question, "what are demons?" You have four ways (arugably more) of looking at this:
1) I agree with Sanoy and SeventyOne. If you do not take the Sethite Theory, Ancient Human Kings and Champions Theory with Genesis 6:1-4 then you have something unusual happening with supernatural entities procreating with human women. (Note: Matt 22:30, Mrk 12:25, and Luke 20:36 are talking about obedient angels who do not marry, these verses do not say angels cannot procreate). According to the Book of Enoch, which the anicient Church and Second Temple Jews considered relevant to quote in the New Testament (Jude, and 2 Peter), demons are the disembodied (dead) spirits of Nephilim (giants). Early Church fathers also believed this. That doesn't necessarily make it right, just shows there was a line of belief stretching from ancient Jews to the Early Church until about the 5th century when belief in angels were attacked by skeptics.
2) Demons are fallen angelic beings which is somewhat a traditional view that overruled the above view that had existed in the church.
3) They are evil spirits that just randomly show up in the Bible with no explanation other than they are antagnostic to God's plan and humanity.
4) They are figments of man's imagination used to explain the various evils in the world.
You asked elsewhere in response to someone else as to whether demons are alive? I would say yes. When we die (if you accept view #1 above) then are we still alive? Yes.
Are supernatural beings alive? Yes, so either #2 or #3 above mean they are alive.
The only way they wouldn't be alive is if you accepted view #4.
With that said, we know what we need to know about them to combat them. They are under the authority of Jesus Christ and through His authority and the name of Christ they are sent away and defeated. We also combat them by getting closer to God, aligning our lives with His will and keeping doors closed in our lives that would otherwise invite them in. We don't necessarily need a report on who they are and what they are to gain victory over them.
I would say you might be worrying too much about the origin of your evil thoughts and desires. In some instances, yes, it's important. If you are being oppressed, it's important to deal with the culprit through the power of Christ. If you are called to help someone possessed, it is important to know they have an evil spirit. It may also be important to know the origin if you need to pray over something like a habitual sin that has you in bondage cause you might need to pray for the link to be broken.
However, overall, where our evil comes from could be a bit of both, it could only be one thing. What we know for certain is they exist, Scripture tells us that. What they are and where they come from is debatable as Scripture is silent on that. We can appeal to ancient tradition as I did above or we can simply rest with the assurance that Christ's name is our weapon against them.
Hope that helps.
Edited: Typed this up to fast while at work. I keep finding typos.