1. Is there a need for employer B to re-file my PERM and I-140? 2. Will my priority date be retained?

Employer B, if a successor in interest, must file an I-140 Amended petition.

What is a b1=b1 visitors visa and what us the process to go through to

US immigration paths to a green card are primarily family or employment based. You should consider consulting an immigration attorney for what...

Will he get green card if he is not 21 at the time my spouse files a petition for us?

Your US citizen spouse may sponsor a stepchild. Please seek the assistance of an immigration attorney to avoid delay.

How can I keep my mom (only family member that I have back home) in U.S. after her visitor visa is over

A US Citizen may sponsor a parent. I recommend you consult with an immigration attorney as to what options may be before you and your mother.

Yes, you may marry a US citizen while on CPT. The US citizen may file a spousal petition on your behalf and you may file a green card application....

Can I apply for B1/B2 visa for business trip ( having my current company justification ) while an approved I-130-F4 brother

Yes, you may apply for B1/B2 keeping in mind the other attorney's advice.

How should I proceed further? what needs to be done on this situation to proceed with H1B filing?

You would provide your employer a history of your US travel and stay. A copy of the CBP report may corroborate your history.

Visa Extension

Although, immediately out of status. A denial notice often states a few days from which you should leave the US

employment based priority dates are available for other employment sponsorship. unfortunately, not transferable to family based cases. or, vice-versa.

H1B extension with same employer in Vancouver, Canada

You are eligible for the interview waiver program in India. You would likely be interviewed in Canada.