ghosts and spirits are the same thing really. myself, i was taught that a ghost is attached to a house or object, a poltergeist is attached to a person, and a spirit is free to wander wherever, but they're really all just spirits. orbs i've usually found are weak spirits, normally animals. i've never seen a ghostly animal, but i've dealt with them. my first dog and first cat for years after they died would sneak into my room and curl up at the foot of my bed. my cat more than my dog though. [he always had trouble coming up stairs, guess even in death he couldn't do it] i would hear a purring noise, and would feel/hear something jump onto my bed and walk over to this one spot [where my cat always slept] and curl up. i'd try moving my legs, and it would feel as though something was in the way. in the morning, there was an indent that looked like something slept there.
your theory that spirits were always spirits is a neat concept. myself, i was told some demons are spirit energies who have never been human. true or not, i'm not sure, never come across a demon.
spirits get stuck on earth for a couple reasons, one [like you said] they're scared to cross over. two [again, like you said] they have unfinished business. third, which is what i've found from talking with a couple spirits, they like it here and just don't feel like crossing over. one spirit i met has been dead for 100 years [more now] she enjoyed watching over people in the house, so she didn't feel like crossing over. another was a great cousin of mine who died before i was born. she loved her home [which is a bed and breakfast she opened with her siblings. still run by our family] she just wanted to stay and see everyone. [because the family would visit every sunday] went back there two summers ago, she's still sitting in her rocking chair in the living room by the window. still doesn't want to leave. the third is Rori, he can't remember being human, he just loves being a spirit. i've asked about crossing over, he wants nothing to do with it.
personally, you just treat spirits like anyone else, they're not so bad. there was a line i heard once that i adore 'I love living in a haunted house, it's like having roommates, only they don't eat my food.'