Work on H1B beyond 6 years with valid I-140

you can stay beyond that date if extension continues to be pending.

Yes you should be able to do that.

You should be Ok but given the intricacies should work with an attorney to get the desired results.

The entire petition. The lawyer who handled the matter will be able to assist.

How much cost to apply for L1 Visa

Attorney fee cannot be discussed on this forum. Need more information to determine USCIS fee. Time frame will depend on where the employee will...

Yes you can apply gc while h1b is pending.

Is it possible to withdraw my visa extension(I-539)?

Yes you may withdraw. That action by itself may not have much impact on your future ability to return to the US.

Chances for getting NIW approved?

You may apply now. Chances of success could potentially increase as you spend more time in the field.

High salary is just one factor in a successful eb1 petition but could potentially work in your favor.

I want to convert my B1B2 visa in H1B visa...is it possible?

Will depend on the role and your professional background.