N-400: Yellow Letter: Does it mean that background check is complete?

The yellow notice means that they are ready to schedule you for an interview after preliminary checks.

My Question is: Can they (current employer) legally take any action against me based on that promissory note?

Very likely it's an unenforceable note but you should discuss this with a contracts lawyer. The big question is "What was the $10,000.00 for?

My maternal uncle is a 73 year old illiterate u.s citizen and he wants to file immigration for my mother(that is his sister)

Your uncle can file the application without problems. The citizenship application does not ask about brother and sisters, only children.

About to finish my OPT period i have on my passport a valid B 1 visa, any procedure to extend my stay under B1 ?

You need to either file to extend your visa by filing an I-539 application or you must leave.

K1 vs K3 Fiancee Visa

Give it time. If you don't hear from USCIS in 3-4 weeks, then get an Infopass with them to check on the status.

Condition green card

Get an experience immigration lawyer to help you. If your ex did send such a letter, it would cause problems but a good lawyer would know how to...

To answer your question specifically, you should open a join checking account and use it. An unused account will only cause suspicion. I would also...

The only way you can legally work in the U.S. is with another visa. You need to discuss this with an experienced immigration lawyer.

I am in USA for past 11 years and have two kids also I am a single mother

There is no way to sponsor yourself. I would seek a second or third opinion from other lawyers. Go to American Immigration Lawyers Association and...

Can a person apply for EAD card or work anywhere in USA if he is on E-1 visa (business visa)

Yes, you can apply for an EAD card if you are an e-1 visa holder.