A desperately lonely woman is unknowingly trapped in purgatory until her grieving husband is finally able to let her go

If the husband is the protagonist that would make the McGuffin. So try writing it from the husbands POV and it enables action to be entered.

A grieving husband must ..... to release his wife that doesn't realise she is stuck in purgatory.

I may even drop the wife's knowledge. Leave it for the story.

If your story does not have the correct components to form a decent logline, it could be that your story is faulty and that sucks. What doesn't suck is that you can use a solid log premise and adjust your story where perhaps the rewrite will save it.

Is the purgatory literal or figurative?