OPT and SSN slight mismatch

As long as the digits match up, there should be no problem for you.

I am planning to file COS from L1A to F1 with a school acceptance date of July 14th (next Monday).

It does not appear that you would have had enough time for that plan to work.

Can i apply to extend my b2 visa as i want to make adjustment by october first and still there is no visa available for me

Wanting to adjust status to permanent resident is not a valid reason for requesting an extension of B-2 status.

The address for submission of that package is available at www.uscis.gov.

Can I apply for an E-3 visa if I am already working for the company as a foreign resident, but just not residing in the US?

Yes, that is an entirely possible scenario.

Status of my H4 visa

You can take classes in H-4 status. You cannot take classes while simply waiting for an F-1 approval.

How to know the applicable prevailing wage once the green card (for employment) is received ?

You can request a copy of the prevailing wage determination our consult with the DOL's OES survey.

Affidavit of support should complete i-864 or i-134 form?

You will need a co-sponsor to the extent that you cannot establish your financial wherewithal adequately.

How to fill out 'Applicant Employment last five years(If none so state)' for the G-325A form?

If the question does not apply, given the response to the previous inquiry, then indicate "N/A," or list your usual occupation.

F1 Status Violation

You have to get a new I-120 and apply for a new F-1 visa. Good luck.