I'm hoping to turn it into a mystery book series ... getting it on TV is only a dream, but of course it could happen. First I have to get the opening book published!

I don't know if it's true what Christie said or not, but I can see how it's possible. Make a bunch of characters suspects, decide at the end, and then go back and seed little clues along the way. Several mystery writers have told me they work that way; I myself knew the main suspect going in, but after finishing the first draft I did go back and fix up the clues a little.

I do have a "grand conclusion" at the end, because my male lead loves that kind of stuff and sets it up. (His wife rolls her eyes at him a lot.) However, I'm twisting that trope of the real murderer being there, just to keep people guessing.