Records of Merlin appear in the 10th century, listing him as a mere prophet. His role eventually expanded into that of the magician, prophet, and advisor to King Arthur. This evolution seems to come from authors rather than hard fact. Merlin was named Emrys at his birth in a place called Caer-Fyrddin. He was known rather by Myrddin, which is a Welsh name taken from his birthplace. The transition from Myrddin to Merlin is attributed to the author Geoffrey of Monmouth (known for inventing and changing names and events to suit his writing), who Latinized the name so as to make it less like the French word, merde, which means "excrement." The name is normally seen as Merlin and seems to be the more correct of the two, but I feel that Merlyn is valid as well. It think at this point it is up to personal preference.
This depends on "which" Merlyn is being discussed. There are 2 Merlyn's, the real one, and the mytholoigical one. The spelling Merlyn comes from the old Druidic spelling and historically speaking is the "real" way to spell it, from tales from Celtic Lore. "Merlin" comes from the stories of British and French writers, the spelling was adjusted to the english alphabet. For further info check out "The 21 Lessons of Merlyn" by Douglas Monroe. Hope this helped clear up some of the century old confusion.
There are a few places like that, I caught it too. I guess it's both, he's a magician, he can spell his name however he wants. :) Seriously, either could be correct, it's just a typo, but an interesting one. if anyone ever finds out how it's really spelled let me know.
Yeah author's perogative and all that, like in Pride and Prejudice Elizabeth Bennet's nickname is spelled as both Lizzy and Lizzie at different points in the story.