How are we going to pronounce this?Oooh - is this Manon? I would go with the first option (Grieux's), because the x is silent in the original French.
Not for me, I'm afraid:How are we going to pronounce this?
Des Grieux's clutches - suggests Day Gree-er-z-zes clutches.
Des Grieux' clutches - suggests Day Gree-er-z clutches.
I'd say Day Gree-er-z clutches so I'd go for Des Grieux' clutches.