'seeing' and 'hearing'?

I put the same question up several forums, but I don't get it well yet.
So I'm putting it up again with some touches.

I really want to know the meanings of the verb 'see' and 'hear in the progressive except for the cases
(and there will be more cases with changing their meanings)-

a) hallucination:
: I'm seeing stars.
b) meeting someone:
: I'm seeing the doctor in the afternoon.
c) listening to people giving their opinions:
: What are you hearing from people there?

Now here are the examples I've collected until now.

1.a. I can't believe what I'm hearing here!
1.b. I can't believe what I (can) hear here!
: I found 1.a. when I was watching an America drama.
'hear' doesn't go with the progressive in the meaning of sounds coming through the ear, AFAIK.
I still feel 1.b. is more natural and I can't seem to find the difference between 1.a. and 1.b.
Is there any good reason for the hearing in 1.a?

2.a. I've been seeing the dark entity that haunts your house and your land.
2.b. I've seen the dark entity that haunts your house and your land.
: 2.a is a line from an America horror movie, and the female main actor did see
the ghost hanged from a tree just one time. 2.b. is more natural to me and I don't know why 2.a was said.

3.a. Can you see what I'm seeing?
3.b. Can you see what I see?
: It might have been 'do you see...?', but I'm not sure. I heard 3.a when I was playing a computer game.
Isn't it good enough to say 3.b.?

4.a. Why am I seeing this?
4.b. Why do I see this?
: 4.a is a message box popping up in Windows to ask if you want it to remember passwords.
4.b sounds better to me. Is there any reason for 4.a.?

5.a. What are we supposed to be seeing here?
5.b. What are we supposed to see here?
: Here's the context. There is a man in the living room with his friends,
and he shows them a video. But they don't seem to understand what the video is,
and one of them says, "What are we supposed to be seeing here?"
What is the effect of saying 5.a. instead of 5.b.?

Is there any reason for the above uses of the progressive? any effect?
I was reading a grammar and found this explanation, which I'm not clear on:
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... it's difficult to catch oneself in the middle of a single act of seeing or hearing.
(though for a scientist not impossible) To see or to hear is to receive a sensory impression.
The reception of the impression is an involuntary act:
we cannot normally prevent the completion of it.
Thus, as we commonly say in English, you see - or you don't see. (fine so far...)
Yet sometimes it's possible for us to perceive the reception of a visual image in an unfinished state.
Wearing the wrong spectacles or having dined not wisely but too well, I might be seeing double.
Such occurrences of see in IU (single act / uncompleted) may be rare,
but they help to explain why see is normally found only in the completed aspect.
Yet in SU (series of acts / uncompleted) see would occur as frequently as any other verb,
since we have no difficulty in imagining an uncompleted series of the acts of perception;
e.g. “I'm seeing too many pictures,” said Sue at the exhibition. “I can't look at any more.”
(I'm completely lost here...)
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Could you elaborate on this and tell me your opinions?