Just putting in my two cents...I worked as an insurance agent for 14 years and just left the industry in April, so this info should be fairly accurate.
The first thing to remember is that all of this info is general - the laws and the coverage will differ from state to state and company to company. As an adoptive parent, the laws of your state (noth the birth state) will be the ones that come in to play. Very few companies will provide coverage at birth since you have not legally become responsible for the child. Prior to actual placement, you have no legal tie to that child, it's a good-faith contract that the birthmother will place her child with you, or for that matter, that you will accept the child. (Let's be honest, placement may not happen for any number of reasons on either side.) Once placement occurs, a company MAY agree to provide retro-active coverage back to the date of birth - again, this will vary from company to company and it's always best to call your hotline to find out. As for the social security number, if you explain the situation to you company they will be able to make an exception. Adoptions do not need to be finalized for coverage to occur - that really should be nationwide. Also, if there are pre-existing health issues (preemie, jaundice, etc) your company should be covering those costs from the time of placement, again, if you KNOW that there will be health issues, double check with your company. But, it shouldn't be a problem since most companies look at placement date just as they would a birth date.
Hope this helps.