I think you should go with the single key, and the fleur de lis. ESPECIALLY because you seem particularly smitten with it! Also because I feel vaguely smitten with it.

I'm actually considering my own favorite key right now (the diamond floral key), and was also pondering whether the key design was too trendy...
...but then I thought that I love this necklace, so why should I care if it's on-trend at any given point in time? I tend to gravitate towards pieces that look vintage-y and whimsical, and if this pendant does it for me, I don't care what anyone else thinks. Besides, the necklace is still made up of Tiffany diamonds.... you can't go wrong with those.
I think you should go with your gut. If it's telling you this is the necklace for you, get it!
As for the chain, I agree with the other posters (16" makes it appear more formal, while the longer chains make it look more casual). I honestly think you could wear it either way, but I'd probably go for an 18 or 20" chain.