My H1b is valid when I requested for cos to F1.I started my classes when cos is pending.Will my cos be denied if my H1b expires?

It is possible to maintain H1B status if you are employed by the H1B employer per the terms of the H1B petition and attending school as well....

Working overseas (possess H4). Can concurrent EAD and L1A applications impact each other negatively?

The H4-EAD application can be filed only if present in H4 status in the U.S. at time of filing. It is possible for both your H4-EAD application...

Under H1B portability rules, the 240 day limit authorizing employment becomes applicable only after I-94 expiration date, and it may not apply...

Not a problem at all if you continue to be a full-time student. When changing residence be certain to file Form AR-11 with the USCIS.

H4 dependents I-539 still in process and their I-94 got Expired ? How long can they stay in US waiting for I-539 approval ?

They remain in an unlimited period of authorized stay with a correctly filed extension application even after their I-94 expires, and no action...

Grace period is 60 days from employment termination, not from when (or if) the employer notifies the USCIS and revokes its H1B petition. It is...

H1B transfer denied but didn't resign to join the new company on receipt. Does this in any way affect my legal status?

No legal implications in this scenario. There has been no status violation.

Do I need to apply for I-131 or Advance Parole?

It is prudent to file I-131 as part of the adjustment of status package. Otherwise if you need to travel abroad, your pending adjustment of status...

Can I Travel in H-1B with GC-AP as a backup?

Yes, entry made with advance parole will allow for future H1B extension petitions with pending I-485 if upon entry employment resumes with the H1B...

You can donate plasma, but payment of any kind (whether called donation or gift, etc.) is not permitted in H4 status unless you have EAD.