I am curious about your view toward multiple POV's...There are 5 POV's. Yes, five: heroine; heroine's husband (murdered); killer; detective; hero. The book goes over two summers....
I edited out some of the information about the book, but this is a fabulous question.
In reality, there is no fixed rule about how many point-of-views you put into a story. Yes there are trends toward POV numbers in different genres but there isn't a fixed rule.
The answer to this one is pretty simple. As you add more and more POV's to your story, the more complex the story becomes and the more you are holding your reader accountable for keeping track of all the storylines. This can sometimes become a difficult task for a reader if he or she isn't reading the book in a single sitting. We have to remember that you will have large gaps of time when we simply don't see a character or hear from them. Add in more POV's and that time period increases significantly.
I think the other issue that comes into play here is how connected the characters are to each other. Sure they may be all associated with the plot, but if they are really working with their own storylines then you have placed more of a burden on the reader to keep things straight.
In many ways, I see the POV discussion much like an art class. If a class is going to put together a display of all their works we will obviously have multiple Points of View. But here is the twist. If everyone is painting the same image placed in the middle of the room, thematically putting the display together will be a bit more unified. These are all just variations on a theme. If, however, everyone paints what ever they want to paint, it becomes much more difficult to see that theme.
Again, you have to remember to do all you can to assist your readers. If you make things too confusing, you will lose them, regardless of how good your story truly is.
Scott