Yes if you have an extension approved and want the new visa in your passport - the consulate will process a visa up to 60 days before it becomes valid. If the question is can you get the visa that is expiring in December, 2012 in your passport so you can re-enter the United States, that answer is also yes.
Only after an attorney reviews all of the facts of your case can that question be answered. I presume your current attorney has already done that and provided you with the best possible option for your case. If you are uncomfortable with the opinion of your current attorney I would suggest paying for a consultation with an experienced immigration attorney. But if you are satisfied with your current counsel and have asked them this question then I would follow their advice. They are in the...
No, you cannot file for employment authorization until you file an I-485 and you cannot file the I-485 until your PD is current. You should consult the attorney that handled your PERM and I-140 with questions regarding the procedures related to that process.
Only the days that you are physically in the U.S. on your L-1B count towards the total period of time. The days spent out of the country can be "recaptured". However, you should have your company's employer review all the facts related to your case including your travel in and out of the United States and make a full assessment as to whether you can recapture any time after your visa expires in April, 2012.
You need to talk to the attorney who filed your extension petition. It could just be an error on the receipt notice. If your attorney has proof that it was filed timely then there should not be a problem.
Without knowing all the facts and circumstances it is impossible to properly advise you. I recommend that you have a consultation with an experienced immigration attorney and at least get your questions answered and learn about the process and any additional options.
I agree with my colleague. You may also want to schedule a consultation with an immigration attorney to go through the process and make sure you have all the necessary forms and documents before filing.