Writing a half-hour presentation about fitting molecular models into X-ray-derived difference density will be a bit of work, but at least getting to Grenoble ought not to be a problem; it's famously a TGV destination, I have all of Sunday to get there (conference starts at 8pm Sunday; this being France it starts with an enormous buffet meal), and all of Wednesday afternoon to get back (conference ends noon Wednesday).
The latest Paris-London Eurostar with seats leaves before the latest Grenoble-Paris train with seats gets to Paris. Easyjet in theory flies to Grenoble, but all its flights are sold out. Easyjet's one daily flight from Lyon leaves too early. BA has (expensive) flights at good times, but there's no train from Grenoble on the Wednesday afternoon to get to Lyon in time to catch the BA flight, even though the Lyon TGV station and airport are conveniently colocated.
I have ended up by throwing up my hands and asking the travel-agent who sorted out my trip to Turkey what she can do.
PS: Miraculous and entirely unexpected resolution by vicarage