If they back out of the promise to have a retrial, I imagine that'll be the last time the USA ever gets any fugitives deported back from France, or indeed anywhere in the European Union.

As for whether a fair trial is possible that's another matter. I wouldn't have thought that a conviction in abstentia would carry much weight with juries that have grown up with movies like The Fugitive.

Of course it won't help that the US Media will be running stories about it from now until and during the trial. Of course that applies to any high profile case, I imagine.