I'm not sure how much traction this idea will gain, but I'm not outright averse to it.
On the technical side, the way I'd foresee this working is:
- When the save requirement is met, NL checks if any zombies are alive on the level. If not, the talisman is awarded.
- When a zombie dies, NL checks if the save requirement has been met yet. If it has, NL then checks if any zombies are alive on the level. If not, the talisman is awarded.
The postview screen is basically UI fluff at this point. Records (including talisman unlocks / completion status) are registered the moment you achieve the requirements in-game, so eg. if you reach a level's save requirement, then instant-restart via hotkey, then exit, the level will still be marked as completed in your save data (because you did reach the save requirement, even if you didn't proceed to the postview screen afterwards). I will stress that it is only recently that NL started fully working this way, V12.10 or V12.11 (I forget which).