Mab can only die on the Stone Slab or on Harry’s Birthday, barring a total breach of reality threatened by the eye which she might have been afraid of in Battle Ground or the destruction of the universe by the White God. Interesting that Mab thought Harry could kill Molly, not that he would. Maybe the Spear?
Also might Molly’s death been a weapon against Ethnui? Molly is still a Wizard what if she had agreed with Mab to use her death curse on Ethnui? Supercharged by the Winter Mantle by a dying immortal it would have hurt Ethnui quite a lot allowing a win for Harry even without Mab, especially given the amount of power required to kill Mab expended by Ethnui. A Mab contingency plan.
The first two are not going to happen during Battle Ground, the last would leave Harry in no position to do anything. Halloween is when reality is at its thinnest so the Eye creating an artificial localised Halloween is not unexpected, Mab could have died at at time when there was an coordinated attack on the Outer Gates being dealt with by Lea. Mab would lose Chicago to save the Outer Gates, if she died Molly became Queen leaving who as the Lady? Molly was NOT ready to defend the Outer Gates, killing Molly would mean the Queen Mantle defaulted to Lea who was ready to defend the Outer Gates. At that point if Mab could die, so could Molly, especially using the Spear. Mab was simply not sure she could survive these circumstances, and it appears to have surprised Ethnui.
Mab will be training Molly henceforth in the defence of the Outer Gates now she has mastered the Lady’s role and performed well in battle (The one big issue with Molly, she was not a combat mage like Harry, and got badly injured during her first big battle). We know Mab has at least another TWELVE MONTHS in which to instruct Molly.
No I don’t think this is foreshadowing of Harry having to kill Molly later on, the Fomor attacked when Winter was weakest, both on the day of the year AND integrating a new Winter Lady. This was Mab making a cold logical assessment that indicated Molly had to be sacrificed for the greater good. Winter are not going to be so exposed after Battle Ground, so another form of attack will be required, Molly isn’t a weak link and this is now apparent.