I concur with my colleagues

How should i apply for I-751? I'm not divorce and not seperated, but not living together.

As you are filing jointly you will need to establish that you are in a bona fide relationship

Permanent green card removal

You cannot do it, however, if you are asking how your mother can do it, she can go to the US consulate and surrender her status.

L1 to H1. Out of status in between. Is H1 not legal now?

I agree with my collegues he is probably out of status as he was in a status that requires employment and his employment ended

What are the chances of working in the U.S. as a non-professional?

If you have an employer willing to petition for you the are some possibilities. You should confer with an immigration atty.to explore possibilities.

Question regarding I-130 question no. 16; "Has your relative ever been under immigration proceedings?"

You should confer with an immigration atty. If immigration determined a misrepresentation occurred there may be a significant issue

How long is the process to get my US Citizenship and Passport after I apply?

US citizenship is taking about six months in most locations. You can expedite pp issuance after US citz

Can I activate L1 after receiving H1B COS approval?

You have a brief period to change your employer. If you intend to delay 2 months you may consider an amended h1b to change the start date

I agree with my colleague

I want to apply for green card. Should I fill under the EB3 category or shall I wait for EB2?

You can establish your priority date in EB3 and upgrade when eligible for EB 2. But this will require two petitions