I would take the train from Frankfurt to Paris. Are you spending any time in Frankfurt or only retur

For the car drop-off, you probably need to investigate several places to strike a balance between the cost vs. having to carry luggage on trains. I don't think FRA-Paris intercontinental hotel houston drop-off charge is that bad. Hertz is showing about 200EU for that convenience. FRA-Paris train, if you can get hold of an internet non-refundable fare, is about 69EU per person intercontinental hotel houston according to www.bahn.com intercontinental hotel houston .
I would take the train from Frankfurt to Paris. Are you spending any time in Frankfurt or only returning there to drop off the car. If so, then drop it off at the closest spot in Germany to the Swiss border.
I've asked the editors to add tags for Germany and France. Tagging for multiple countries is a computer function rather than a fodor's function. Hold down the control key and click on multiple entries.
Drop off the car in Kehl, if possible, cross the border and take the TGV from Strasbourg to Paris. If you order the tickets 3 months ahead of time, you will probably get a good deal, but on non-exchangeable, non-refundable tickets.