The below is not intended to be legal advice, and we do not have an attorney-client relationship -- if you accept then keep on reading.
*Two things are blaring in my mind - why isn't your lender requiring one? Maybe they THINK they do not need it, but will inform you a few weeks into the process that they do need it, especially if they are transferring it to Freddie/Fannie.
*Maybe the Seller does not want to know the Phase I results, since it may trigger certain notice requirements, if he himself has a loan on the property. (i.e. if he is aware of certain hazardous environmental conditions this may trigger a duty on him to tell his Lender, or else the Lender may place him in default of his loan). If Seller has a loan, might be a smart idea to see who the lender is - because if its a national lender, then I wouldn't be too worried, since they've gone through the whole underwriting process and evaluated the risk associated with the tank. Also, maybe the Seller could provide you with an old Phase I (if Seller doesn't have a copy, he can easily ask his Lender, or have your attorney ask -- the Lender's attorney usually keeps a copy in their file).
*What if you told the Seller "look I understand that you do not want to pay for the Phase I, but since we agreed to this before hand, I will order the Phase I and it will be paid out of the closing proceeds when we close, and if we do not close, then lets agree to split the costs with respect to the Phase I."
*what does your "default" section say with respect to a default by Seller? & What are your thoughts on an Environmental Indemnity, so that if anything does arise, Seller, and any principals of seller (so that you have a real person on the line - and not some entity whose only asset was the property being sold) has the duty to indemnify you for all costs. (this indemnity is only meaningful and worthwhile if the person you are going after has enough $$ to cover your losses - hence i said make sure you get a real person and not an entity).
I didn't read the entire thread - but wanted to pop these little tools your way.
Excited for you and your wife - best of luck!