I think it depends on the rest of the marital evidence that you CAN submit. If you really don't have much financial evidence, then yes, wait. But if you do have solid evidence, I don't think a tax return will make a difference.
Neither. It is based on the expiration date of your green card.
Not really, but getting an extension letter is proof of your status should you need to travel. So, to me, the earlier, the better.