Gen.3:24
I see this as a job description for a couple of angels. I believe the garden of Eden was a real geographical location on the Earth. I realise not everyone does but Adam had to be stopped from going back in and eating from the tree of life. Consider how depressing it would be if we lived forever as sinners? Like I said earlier, our own Lord serves, why should any creature be different? Consider natural biomes and how they serve each other and what happens to them when "unnatural" events are introduced. "I came to serve, not be served" I believe is the mindset of the Lord we worship hence, creation itself when working within His will, also serves.
Col.1:16
Everything including angels were created by, for and through the Lord Jesus Christ. Why create angels? Why create dogs and cats or cows and goats? They do similar things yet fill different needs. I guess really what I'm saying is why not create angels? Are there aspects of the creation whereby their uniqueness lends itself to a job requirement that we cannot do? Does not the extensive variety of the creation fill you with a wonder and joy that would be missing if there were only a handful of variations? I dfor one love finding new things.
Neh.9:6
Job 38:4-7 suggests they were created before the universe.
“Where were you when I laid the foundations of the earth?
Tell
Me, if you have understanding.
5 Who determined its measurements?
Surely you know!
Or who stretched the line upon it?
6 To what were its foundations fastened?
Or who laid its cornerstone,
7 When the morning stars sang together,
And all the sons of God shouted for joy?
Heb.1:4-14
This passage is distinguishing between the role of the Son and of angels. Not sure why you would ask if they are free to sin on the basis of this passage but Lucifer sinned and unless you want to say God forced him to sin (and I'm sure you wouldn't), then I think the sin answers your question. They are spirits who serve. v.7
2Pet.2:11
I think I have addressed this. It doesn't say superior, it says greater. A person can be greater in power and position but that does not mean greater in worth or value. Your president is greater than you but he is not more valuable than you. That's why he gets the security detail but you don't. It's because of his power and position, not his inherent worth.
1Tim.2:3-5
I think the verse answers your question. Only
one mediator, the man Jesus Christ. I don't know of any verse that describes them as mediators.
Ps.91:11-12
Why did God appoint teachers? Isn't the Holy Spirit suppose to be our teacher? If through this post you find some encouragement and/or comfort, have I become superfluous? After all, is not the Holy Spirit our Comforter? Before time began and anything was created, God thought to share His life.
Titus 1:2 If His life is love and giving (and it is) should His creation be deprived of that same life? Why do you serve the Lord, out of fear of punishment or love of who He is? I'm pretty sure you would answer the latter. He has set eternity in our hearts and the desire to do as He does should not surprise us. Even Jesus said He only did and spoke what the Father showed Him.
Lk.2:9-15
Well, we can say there is a lot of them and they abide in heaven.

God is not wasteful and He knows what He is doing. I believe if He wanted us to know more He would have told us more. There is always the danger of
1Cor.8:1 with much knowledge and while the wise seek knowledge, humility teaches them their limits.
As to their own salvation history? It can't be like ours because they started from a different position so I think trying to make comparisons is futile. We know God is merciful. Lucifer sinned long before we came on the scene and God created the lake of fire specifically for the devil and his angels
Matt.25:41 but they are not there yet so, I think that tells us something of God's grace toward the rebellious spirits.
The forum is playing up on me again so will answer your last paragraph in another post.