There seems to be a bit of a contradiction in what you’ve written. On the one hand, we are to ignore the writings of the Celtic Christian monks and scribes and dispense with awen as druidic concept, but on the other hand we have to see Ceridwen as a goddess for which there is no evidence as such, other than those very writings which we are to ignore? The other difficulty is that the Welsh tales were written down in a time when Ireland was nigh on converted anyway. In fact, the Irish were historically the bigger colonisers of the western parts of Britain at that time, hence the ogham stones found in Britain.
We should also be careful about dismissing out of hand what the Greek and Roman writers wrote. Inasmuch as we need to take everything with a pinch of salt, at the same time, just for example, the only real evidence we have for the associations with mistletoe and oak come from those same writers.
I’d also be careful with interpreting megaliths as specifically druidic, I believe that there is no way that the ancient peoples of these islands would not have noticed these imposing monuments, but at the same time they pre-date Celtic-speaking cultures in Britain and Ireland by millennia. There’s not much to suggest that they were specifically druidic.
The Welsh, Cornish and Breton languages are the direct descendants of the Ancient British or Brythonic dialects that were spoken when the Romans arrived and they are related to Gaulish. Curiously enough, the Celtic and Italic languages seem quite closely related within the family of the Indo-European languages and the farther back you go, the more similarities you can find.
Remember too, that there are different threads of Celtic spirituality and culture and the bardic tradition is the one that really did survive the travails of time, war, conquest and change right up until the historical period – albeit hanging by a thread. I don’t think we should dismiss these traditions as they are still a part of living Celtic cultures and very much integral to the survival of the Celtic languages, most of which are in a pretty desperate state to be honest.