@Harel13 hello
Here are the Bible's answers to your questions.
1. If Jesus is an angel and is also defined as a 'god' - does that mean that all angels are 'gods'?
Is there any room to say that various pagan gods are also powerful beings in this sense (bad, good, whatever)?
Jesus - or more specifically - the word (Michael) is a son of God.
Like his brothers - the heavenly angles, who are also sons of God, Jesus is a godlike one.
Hence, Michael and the angelic host are gods. This would include angels who turn bad.
An angel's attitude in no way makes "him" not godlike - that is, a powerful spirit being.
Pagan gods are no gods at all. They are called gods by idol worshippers.
From this I gathered that: In order to reach heaven, a spirit needs a spirit body. To remain on earth, a spirit needs a physical body.
This a misunderstanding of what is being said.
What lives on earth, is fleshly, or physical. It is not a spirit living on earth, but a physical body, which is given spirit, of life force. In other words, the spirit is the force of life, it sustains the life, or soul. When that life ends, the spirit is no longer useful to the body, and goes out. It then "returns to God", in that God can give this life-sustaining spirit again... if he wishes.
Beings in heaven have spirit bodies. I am assuming this is what you are saying.
If pre-mortal Jesus was an angel, i.e. a spiritual being in heaven that was injected into a human body, that means, presumably (correct me if I'm wrong), that when he died, his spirit should have returned to heaven automatically as the archangel Michael while his body stayed behind. Yet JW teachings are that god didn't leave Jesus' body to rot, so he resurrected him.
The life of the word - Michael - is what was planted in Mary (Side point : A good scripture to show that life begins at conception, from God's viewpoint).
So the spirit body, the word had in heaven ended, and he would now have a physical body on earth.
Think of the resurrection of those who would instantly receive a spirit body to receive their heavenly body. Paul said, they do not "sleep in death". They transfer is instant - instant sleep. Instant wake.
This is the same with the word - instant sleep. However, he did not wake until he became a fetus... Well at least partially awake. You know what I mean. Nine month after, he was fully awake.
Which means: a. that a heavenly spirit (Jesus/Michael) that is meant to dwell in heaven and is defined by that fact that it/he already has a spirit-body, received a renewed physical body. b. that a pre-mortal spirit-being didn't, in fact, return automatically to heaven post-mortal-life, and needed further divine assistance.
On earth, Jesus is fully human - having a completely physical body. He is not a spirit in a body, as I explained earlier.
So when Jesus died, he was not a spirit, but as the scriptures say, he was raised up (resurrected) as a spirit - that is, he was given a spirit body, when his life was restored... on earth.
Like his powerful brothers - the angelic beings, he was able to appear in human bodies, and disappear.
He returned to heaven with his new spirit body - immortal, and greater than formally.
2. If Jesus went to heaven as a spirit-being (that same being that had already existed in heaven previously), why would he need his physical body?
If he didn't have his physical body, does that mean his body did, in fact, rot into the earth?
If he did have his physical body, does that mean he had two bodies simultaneously? How does that work exactly? Does he exist in two places at the same time?
I hope I clearly addressed these questions above, but to answer.
The spirit being that returned to heaven, was the word, with a new body. It is an immortal body. Contrary to popular opinion, the angels were not created immortal. They do not need to eat, or breath oxygen, as humans do, to live, but they can die, and those wicked ones will. However, Jesus in his new spirit body, cannot die. Neither will those who receive heavily bodies, after they prove faithful to the end.
Jesus did have a physical body when he died, but it did not rot in the earth, as the scriptures prophesied.
What happened to his body? Satan wanted to know the same thing about Moses' body. However, he will never know, and God saw no reason to reveal knowledge he has regarding Jesus' body.
After Jesus was resurrected - according to scripture, he was spirit. He did not have a physical body.
JW do not teach that Jesus had a physical body at resurrection, so you might have to ask those who teach such, that question. I think Mormons do, but I think they believe that physical body is spiritual also. However, I am not sure, and speak under correction. Perhaps
@Katzpur can help us with that question.
3. If pre-mortal Jesus was a being who lived well enough in heaven, then was injected into human form, then I would surmise that upon the death of the body, that same pre-mortal being (who is now post mortal) would simply automatically go to heaven (as I wrote above). Why then, the need for godly intervention?
I hope you understood what I explained above. Jesus could not resurrect himself. He was dead. God is the source of life.
However, he has given resurrected Jesus life within himself, so that Jesus can give life.
Sorry, I didn't insert scriptures. I am assuming you know what the Bible says. However, if anything puzzles you, please feel free to ask.