I don't have so much fragrance experience, but I'd be truly surprised if your (reasonably) clean finger could add enough of anything to change a fragrance.
The alcohol should be plenty to extinguish any bacteria. I can't believe the splash bottle form would have survived if a few finger dabs could change the fragrance.
I do suppose it's possible that a splash bottle could get enough air mixed and exchanged inside to cause some ingrediants to oxidize, but again, it seems like it would take longer than a year and what, two dozen wearings?
Goodness, I just re-read your post and it's only three months old! I can't speak with any authority....but I think it would be impossible.
Did they tell you to use cotton when you bought it???? That's gonna soak up a lot of perfume! I hope you stuck in your cleavage afterward....
Oh, I would advise against leaving it under the seat of your car all summer. One ounce of Chanel #5, absolutely cooked with quite a bit of sediment to show for it.