There is no Phantom mayuri, that is just a dream he had, maybe a premonition, but there was no actual reality to it. That would mean Mayuri could just "speak" to him Over different time lines in which case she could explain a whole lot more to him then what she has.
As for the Kurisu thing, the only other message he has to change is the shining finger message, he doesn't have to change -all- of them, so keep that in mind. Not saying there won't be that point, just pointIng out that as things current stand, that decision won't be nessacry within the current actions being taken to prevent Mayuris death.