Apparently none. Of course you can tell stories, but it's not as if we know exactly how grief has (or hasn't) benefited humans from an evolutionary point of view. We can't create genetically modified humans who don't experience grief and then rear them in a parallel environment to see if they have more or less reproductive success than regular humans. So who knows?

Nor is it safe to assume that any existing trait must confer an evolutionary benefit. Evolution isn't a perfect optimizer, and it's an ongoing process without a terminus. All kinds of traits in the current human population might turn out to be evolutionarily disadvantageous and scheduled for elimination via natural selection in the long run. We won't be around to find out.

No way to know, but the concept of "momento mori" https://en.wikipedia.org/wiki/Memento_mori means the living might live more "examined" - and therefore more valuable or meaningful lives - when they are conscious of the inevitability of their own death. Perhaps this consciousness then benefits the society/group they are a member of?

None directly afaik.

But attachment has immense benefit for social animals. Attachment is maintained by both positive and negative emotional incentives.

Grief is the negative emotional incentive ramped up to the max.