My thoughts would be that she'll not make it through the night in any case. Having another cause of death waiting in the future doesn't mean the earlier ones aren't as effective as without. That's not how death works.
(And, actually, if she did somehow survive, the son caused her a guaranteed death date later.)
But of course it's magic, so all bets are off unless you know exactly what the rules are and how to lawyer it.
That doesn't make sense at all. It could easily be some kind of poison curse that takes 7 days, that doesn't mean if they get stabbed they'll just be invincible, or if they die that the curse didn't work, it just means something killed them sooner.
It doesn't have to be an all powerful curse necklace that literally changes the entire universe to make you avoid death under all circumstances for the next 7 days.
Well the all powerful thing is kinda how curses work in fantasy settings usually. The witch says it'll kill them in 7 days so it kinda has to. Maybe the universe will find some easier way of achieving that. Like reviving her in the grave just to instantly die again or something.