The topmost item is the TopContainer - you may get it like follows:
CarouselItem topmostItem = this.myCarousel.FindCarouselPanel().TopContainer as CarouselItem;
Consequently, you may verify whether the clicked item is the top container.
Considering your second issue, if you want the path of the carousel to be a horizontal one, you may follow the approach illustrated in this forum thread
Let me know in case you need any further assistance.
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