Everybody on this forum wants the best for you and a lot of people on here have way more knowledge than the officials who are supposed to be able to answer your questions, unfortunately the officials don't always know the answers.
I really think your best bet is to both fly back to the USA, have a quickie marriage in a civil courthouse ceremony and then apply for a spousal visa. Which Consulate you use will depend on where your G/F lives permanently in the USA. Having a quickie ceremony here in the USA won't take anything away from your big wedding day in the UK. How about you both take a trip to her home and book a courthouse ceremony and while you are there apply for the Spousal Visa.
The other route is to have her fly back to get the Fiancee Visa, (it shouldn't take too long if you have all the documents you need. Take the advice of others on here, they know what they are talking about. You will end up a LOT MORE UPSET if she gets refused entry into the UK from France, it is not worth the risk. She will be left in a foreign country having to pay for accommodation while she gets a flight back to the USA anyway which could cost her an arm and a leg if the ticket is bought at short notice.
Go the easiest route, you will be glad you did. It will mean having to be apart for a few weeks but it will give you less of a headache and less potential of heartache if she is refused entry. You may not have to be apart hardly at all if ever if you can get enough time off at your work to go the Spousal route and go back to the USA with her.
Fiancee or Spousal Visa routes are very straightforward if done right from the start and there are loads of people on here who can give you advice on those routes. If you are in a hurry though, hiring a Courier is the best way to go and will on average cost approx $100.00 +the cost of the Visa.
Let people on here know where your Fiancee lives in the USA and some others can guide you to the appropriate couriers and which consulate to use.
Read this as it will help you decide:
http://www.ind.homeoffice.gov.uk/ukresidency/eligibility/partnersandchildren/fianceeorproposedcivilpartner/And this:
http://www.ukba.homeoffice.gov.uk/ukresidency/eligibility/partnersandchildren/husbandwifecivilpartner/