Welcome to the forums, sudipadcary.
Please ask unrelated questions in separate threads. For one thing, you'll get quicker answers that way.
Here's my answer to #1:
They are both correct and there's little, if any, difference between them.
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Ask#2 under the title Events in rapid succession, and #3 with Had only just been, with sample sentences.
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As English is your native language you should be able to make a reasonable attempt at #4 yourself. Have a try and we will comment. Suggested title: As soon as+past/past perfect.