Yes, I can see the faulty reasoning here: its in your first two sentences. Leprechauns (or trolls, as in Iceland) have always been and remain fictional concepts. It is why they only ever appear in such things a fairy tales and legends. Even those who 'believe' in them acknowledge that.
Conversely, religious issues such as the Virgin Birth or the resurrection of Jesus are reported as fact by people who were either present at the time or who knew of such eye-witnesses. This tends not to be the case with Hindu or Buddhist holy documents as they were not written down for many centuries -
Which real person who was there reported the virgin birth as fact?
'Corruption' over this length of time is very different to that over a matter of 2 or 3 decades.
With the Nativity stories, we are not talking about corruption but fiction.
Anyway, that's all a side issue. The point of the leprechaun thing is that all the same arguments you use for God (especially the negative proof fallacy) also work for leprechauns. We all agree leprechauns are definitely fictional therefore the arguments you use for God must be worthless.