Actually, all of the examples are a little weird. Need is very rarely used as a modal any more, and it's likely that you'll be able to go your entire life without ever encountering it in speech. It is sufficient that you be able to recognize and understand sentences like the above, but don't expect to use them much. Just treat need like any other verb:
Present: don't need to
Present perfect: haven't needed to
Simple past: didn't need to
Past perfect: hadn't needed to
That said, the three examples are grammatically correct, strictly speaking. The phrase needn't have done is a fine, though archaic, way of saying didn't need to have done...
when you say “I wrote my article yesterday,” does this imply that at this moment you have a finished article or not?
Yes, you finished it. Otherwise, as you say, you would say something like "I started to write my article yesterday". In that case, 'started to write' has finished even if the article hasn't.
What about present perfect? Does it imply completeness?
No, it doesn't. Present Perfect is 'until now' or 'relevant now'. It says nothing about what happens after now, though context might. For example:
"I have worked in that factory" implies (by using 'that') that you no longer work there. Maybe somebody is asking you what is inside, and you have relevant knowledge because you have worked inside the building.
"I have worked in this factory for ten years", but this is your last day. You have worked here until now.
"I have worked in this factory for ten years", so you know where the toilets are and don't need telling. Unless you get sacked you will continue to work here.
do I have to use past continuous to emphasize the incompleteness of action?
If the action is incomplete, you use present continuous. Past continuous is for a finished action.
In your example “I was watching this film”, the action - the watching - is complete. You are not watching it any more. Whether you completed the film or not doesn't really come into it, as the film is not the action.
Best Answer
Both are correct, but the second sounds more natural, at least in American English.
There is a very slight difference in meaning between the two sentences. The first sentence emphasizes the speaker's pride, whereas the second sentence emphasizes the act of meeting the listeners. The second sentence therefore sounds slightly more humble.