A1. When it is obedience to Allah's order, it is for God. Hence, because the prostration of the Angels to Adam (as) was obedience to the order of Allah Almighty, it is prostration and submission to Allah, not the prostration of worship of Adam (as). Allah (swt) has the right to say do and do not, and we have to obey him, and to do so, and have no choice for objection.
A2. It was not a reward for Satan for disobeying him, but for his former worship, because Allah Almighty does not waste good work. On the other hand, it is a test for the sons of Adam, and there is no injustice to the human being, Satan does not and cannot control or force them, except if they want to be controlled by him, because Allah Almighty warned them, and forbade them to obey him, and explained to them everything, and made it clear that who follow him is the loser.
Let me give you an example:
If you warned a person not to go on this way, and made it clear to him that there are thieves and murderers, stealing from their passing and killing him, but he did not listen to your warning, and got stolen and killed, are you responsible for what happened to him? Another example:
If you know that you must sit an exam at the end of the school year, and if you do not study, you will fail, repeat the year, and may be expelled from the university or school, and in spite of this, you neglected and not studied, and expelled from school, who is responsible, aren't you responsible?
Q3 answered in A2.
A4. This is a misunderstanding, Adam's eating from the tree did not cause immorality. It was a fall in the test, and he bear the consequence of the fall.
We are neither responsible nor accounted for what he did. We are responsible and accounted for the actions we do, and we know we are forbidden to do, and we have the power and strength to leave, But due to our bad choice and following our desires, do it. You see, if your father forbid you doing something, and he told you clearly: do not to do this thing, and if you did, I'd punish you, and if you did not, I'll reward you, and you deliberately did what asked you not to do, and disobeyed your father. Do not you deserve the punishment?
We see this in our daily lives, the violation of the law causes the offender to pay a fine or to enter the prison, for the offence which he is capable of not committing, is the judge considered unjust if he imprisoned him for his doing?