meaning - 'See' and 'Hear' in the progressive?

I'd like you to go into details about the difference between 'see', 'hear' and 'seeing', 'hearing'.
I'm not a native speaker, so it's a bit hard to understand this explanation that
'see' and 'hear' can be used to highlight
the active, immediate aspect of 'seeing' and 'hearing', (Someone has told me this)
because I think that 'seeing' and 'hearing' can't have duration.
I mean, they're moments, and so we can stop watching, looking and listening
but we ourselves can't stop seeing or hearing something, can we?

So 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?

I understand that 'see' and 'hear' with 'can' means something like other verbs in the progressive.
I've been thinking hard about the following examples and here are my takes.

        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.
  Up to now 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? Exaggeration?

        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 just 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.
  Someone has told me that 2.a. would mean that she'd happened to see the ghost a few times.
  Is it better to think that it is highlighted the fact that she was in the state of seeing the ghost?

        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.
  I've been guessing it might be a change of meaning; watching or something like that.
  I don't know why, but it wouldn't be good to say,
  "can you see what I (can) see?",
  maybe on account of saying 'see' and/or 'can' double.

        4.a. Why am I seeing this?
        4.b. Why do I see this?
: I don't understand this the most.
  4.a is a message box that Windows pops up
  when it asks if you want it to remember passwords for websites.
  4.b is better to me and there seems no reason for putting emphasis on the activity of 'seeing' here.

I've put up this question on some forums and here are the opinions;

(A) Your analysis of the verb see is correct.
       However, 'we're seeing' this use of the verb
       recently to indicate that it is something going on recently
       that hasn't been the norm in the past.
           a> I never used to see him on the street alone,
           but I'm seeing him a lot more often, now that his wife has died.
           b> I'm looking at the same painting you are,
           but I'm not seeing all the symbolism that you are.
       I feel I haven't explained this very well,
       so I hope someone else will chime in with more help.

(B) Hmm. It depends what you mean by "normally".
       "see" in the progressive doesn't seem so abnormal to me,
       especially when "see" is not being used literally.
         Why am I seeing this? ~ Why is this being presented to me?
        If the message said,
         "Why do I see this?"
       it would give the impression that you regularly or often see it,
       i.e., that it is regularly or often presented to you.

Some more good context and practical explanation on the differences
between the pairs
would help me to get a fix on this interesting usage.