Can I bring someone to my green card interview?

Yes, your boyfriend may sit outside the interview area. That is generally acceptable.

How can I change an error of date of birth on my green card?

Go to www.uscis.gov and make an INFOPASS appointment with your local USCIS office. Explain your situation. Take a copy of the letter you send to...

I751 waiver is pending over 3years I’m got married again.

The first waiver is pending, So, you need to try to move for adjudication of this waiver before you can file a new petition.

What should I do if my employer lay me off before my H1b period starts ?

Please provide more details of your current status.

Wish to convert from H1 to F1. Currently completed 3 and a half years in H1 . Is it feasible to go to Canada for H1 to F1 ?

The problem is dual intent. Its likely you may not be granted, however, for H1b and L-1 you may have dual intent. So give it a try.

My I-20 is terminated but I just get married

No i don't see any issue with exception if your adjustment of status is not granted because the marriage is not bonafide.

Can Canadian buy business on H4 and apply later for E-2 ?

The short answer is yes that is possible so long as you do not get involved on a daily activity or work for income.

If person is detained and has scheduled individual court hearing, gets notice that interview is rescheduled ?

If the proceedings are not set for hearing far away, then I would be ok. However, if it is month or months away I would file a habeas corpus with...

As my colleague correctly suggested, do not trust the company adjuster. See a lawyer to represent you now.

Can I get a fair salary?(J1 visa)

You need to hire a lawyer to enforce the contract and sue for the damages. You may report this to the department of Labor and Equal Employment...