Unfortunately, Merlin's character doesn't really line up with anything druidic until the 18th century. By the time the Roman empire collapsed, they had wiped out the druids completely, and all that was left was Roman propaganda, secondhand histories, and Celt myths and stories.
The Merlin character didn't show up in literature for another 700 years, about a century after other Arthurian stories started showing up.
The entire set of Arthurian stories are a result of real histories in Britain after the Roman collapse and Celt and pagan myths being written down in the 1100's.
Because of modern memes and stories, it's hard to grasp how thoroughly the Romans obliterated any and all knowledge of druidic history. The notion of nature worshipping powerful shamans is a modern conceit, and doesn't match any of the known written or oral histories.
Romans went scorched earth on religions they didn't like, and there's probably a dozen or so cultures like the druids where all we have left is bits and pieces of hearsay and mythologies.
Wasn't it more Christianity than the Roman empire that destroyed Celtic/Druid culture (noting that from the 4th century on the Roman empire was itself Christian) ? Druidism also disappeared in Ireland which was never invaded by Rome.
As far as I know the pre-Christian Roman Empire was very tolerant of foreign religions as long as they reciprocated by respecting the Roman religion. I'm not sure why Druidism would have been unique in that regard. Do you have any references supporting your assertions and explaining why Druidism would have been singled out among other pagan religions for persecution ?
