Is that legal to file H4 by the principal petitioner himself for the dependent?

Neither. They will file for their change of status to H4.

The various local offices have different wait times for the interviews. Anywhere from 1 - 4 months.

Work Authorization between L2 and L2 EAD.

You should wait for the EAD to work. Not sure what it means to be on payroll if you are not working.

I have an E1 Interview at the embassy in London next Monday. What am I likely to be asked?

Prove that you and the company meet the criteria. Check the embassy website in case they have special requirements.

How i can change my B2 status to F1...i want to apply for masters degree...how many days it will take to change my visa status..

It takes 3 - 6 months to do a change of status to F1. You would be well advised to see a lawyer first.

If the guy is on OPT can the spouse ( currently in US on academic H1) go on a dependent F2 visa?

Yes, the spouse of an F1 on OPT can apply for an F2.

How many days do I need to complete process of filing i131 and do a walk in to a ASC with fingerprint notification?

Normally you can get printed very quickly after filing for a reentry permit, about 1 -2 weeks. Many ASC's allow walk ins.

Is it possible to enter a college, if I lost my status by staying longer than 6 months in the States?

It's up to the college, some colleges admit people regardless of their immigration status.

H1B transferred by new employer now don't want join and want to continue with current employer.

It depends what kind of a visa you have. If you are transferring your H1b, you should port to the new employer.

BIA sustained my asylum application appeals and found me eligible for asylum and remanded my case back to the IJ who DENIED

Possibly, or you can refile for EAD based on being an asylee. In addition, you are permitted to work without any EAD incident to your asylee status.