Once your Employer A revoked his petition, all of the corresponding approvals(including the stamped VISA on your passport) would be void. You must get the new stamping done before entering into US.
Q1: Is it madatory to go for stamping/dropbox?
Q2: If not will there be any problem at the port of entry?
Q3: My spouse doesn't have approved i539 (for H4 transfer/extension from Employer B). If stamping is required for us, Shall we go for H4 dropbox/stamping with just my approved petition or is it mandatory to have i539 approved document?
Read my above comment.
Best is to apply for both of you and your spouse's stamping together using the new approval.