Yeah, I totally expect Thrawn's story to be largely re-written in the new canon. I was just throwing out a possibility for where (as yet) it may not contradict Legends.
That said, I tend to think that Grand Admiral isn't the kind of position that one survives demotion from, even if you're Thrawn
That's quite true, but if the demotion was followed immediately by events which validated Thrawn's disobedience that might be enough for a stay of execution. It might also explain why the second promotion (as I'm going to deem it in my head-canon for now) was kept such a secret - not because Thrawn is non-human but because it showed the Emperor to be fallible.