Killing someone who heals with stormlight would require not just stabbing them through the body, but making sure you got the heart or something. I think the killing scene had "making sure she is dead" just stabbing her again, not making sure it is something that would kill a radiant. haha.
And she never used a sprenblade for the same reason Shallan after realizing she could, doesn't do it, a girl with a shardblade isn't something you tell people. She keeps the fact that she soulcasts with out a caster a secrete, she isn't going to let people know she could summon a super shardblade.
I think either Ivory saved her and she didn't know she could do the shardblade until after her death, or she needed to fake her death there in an attempt to make sure they stop coming for her and don't accidentally kill shallan like they almost did.