1. Some believe the astral is your imagination while others do not. Try projecting and make up your own mind. the belief is there are different planes of being, we are in the physical, and the astral runs beside us. Personally its real, but arguments can be made for both sides.
2. Again, some will argue yes or no. Personally, yes. I have dealt with spirits my whole life, so for me I know they're real, but some don't belief or want physical proof, so again, draw your own conclusion. A spirit vs your imagination there's a few ways you can tell the difference. The biggest way is if a spirit talks to you, you will hear a voice like someone is in the room. Granted you might want to rule out mental disorders if this happens frequently. The spirit is its own being, so its not part of your subconcious, but an energy form would be you pooling your energy to create a being of energy to help you. Once you stop feeding it your energy though, unlike a spirit, it will go away.
3. Well you can call on a god to join you in your circle, for help, guidance, spell casting, or other means, but you should hold off until you are more advanced in your studies and have a connection with a certain god. Deities are more powerful than a spirit, angel, or demon, without the proper understanding you could get hurt as different gods have different personalities and could take offense from something you say or do even if you didn't mean any offense.
4. Yes and no, we are physical, they are ethral or astral, but we co-exist on a parallel.