Comparison, tense

1.Is there any difference between "present perfect tense" and "present perfect progressive tense"? E.g. "I have lived here for 6 years." Vs. "I have been living here for 6 years." I think they are almost the same?

2. I think the difference between "all morning" and "this morning" is: "all morning" means like, doing something the entire morning. It is is a period of time. Whereas, this morning is just what time something was, it is more instantaneous.
Then, let's see some questions: We can say: I have been writing six letters all morning. So can we say: 1)I have written six letters all morning? I think we can say that, because I have been writing six ltters all morning = I have written six letters all morning. (As i asked above) 2) Can we say: I have been writing letters this mornig? I don't think so. Because you can't use this tense along with "this morning", as i defined "this morning" above. However,I think it can be used along with "all morning". 3) Can we say: I have been writing six letters this morning. I don't think so, either. Same reason as last one's. 4) Can we say: I have written letters all morning? I think so. It's like the first example. Is any difference between "six letters" and "letters" in the examples above? I don't think they have.

3. What does "I have now written a letter." mean? The problem is: What does "now" here mean?

THanks for replies.