Can I apply for K1 fiancee

Yes, if you are a US Citizen and have met her in person within the past two years, among a host of other requirements.

Do we need to withdraw I-539 if the applicant left the country before an RFE

It would not hurt to send a letter to USCIS indicating that the application is withdrawn due to a change in plans.

Participate in Clinical studies while on B1 visa

This could be construed as "working" in the US. be careful.

You cannot continue working lawfully without obtaining an extension of your work authorization.

L1B to H1B Transition

There is no "concept" of ghrance period that would allow you to remain and work without the proper documentation. Avoid a serious mistake and...

Can a employer in the US, hire a Chinese Immigrant on H-1B Visa for a sales role?

You could report any apparent violation to the DHS or DOL.

Does the CSPA apply for all visa categories?

Yes, the CSPA applies, but will only save them as "children" if their biological age, less the period of time that elapsed from filing the I-130 to...

New employer filed h1b transfer before I-94 expiration. Its in process still after I-94 expires and Can I join him?

This is a complicated question, which is best viewed as a primer for a consultation with an attorney. I would not make any moves based on any...

got a Call-in letter to the ICE office

This probably regarding your DV case from 2009. Contact an immigration attorney.

There is slightly more to it than that. Consult with an attorney.