We see Semirhage, Be'lal, Rahvin, Asmodean, Aginor and Balthamel die on screen... we see Lanfear and Ishamael reborn after their seeming on-screen deaths, but the only FS we questionable death up until now was Sammael, who had a similarly off-screen death. There are many options that are plausible for Graendal's escape. Far fetched or not I do not think we have enough evidence at this point to definitively say she is dead.