Can I either file a lawsuit or initiate a collection activity against an US expatriate on a business activity happen in India

Your question is not necessarily related to immigration. Perhaps someone in Contracts/International law may assist you.

Do I need to file tax return for year 2014?

You may need to talk to a tax/accounting person concerning this; whether your income gets to the minimum threshold requiring you to file a return.

Can green card mailed early even on the website it says 60 days?

Yes in most cases it is usually received well before 60 days.

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G325A, Additional Sheet of Paper??

You can use an additional sheet to provide the required information. Provide your name and A number if any on the additional sheet.

Just a question, what are reasons why the USCIS would need more evidence?

Usually they won't send an RFE where the submitted evidence is sufficient to establish eligibility.

Will I be approved for F-1 visa if I overstayed when I was under 18?

Due to your age when you departed, you did not trigger the 10 year bar and so that by itself ought not be a reason for any denial.

I was convicted for a firearm about three years ago , which was drop to misdemeanor will this affect me applying for citizenship

Take the charge, plea, sentence and all conviction records to an immigration attorney who'll then analyze the conviction and advise you properly.

Can the green card of my minor children (13 and 15) be denied due to overstay

There's never a complete guarantee with anything. Your assessment is correct in that the 10 year bar will not impact them when they depart due to...

Does it matter how much money you are making when applying for citizenship?

It shouldn't matter unless you have other concern.

1. How long I have to wait for filing my citizenship? 2. How long it take to bring my wife from India, I have 10 years GC?

You'll have to wait 5 years or 4 years 9 months to be precise.