Some great research you’ve done there, and some very good questions raised.
There is also the aspect of a distinct Celtic Church being in Britain before the Roman missionaries arrived with their own version, which caused arguments between the two, such as the dating of Easter and the difference in tonsure, and the practice of penance, amongst other things. In reference to the the Easter date issue, at the Council of Nicaea in 325AD, it was agreed that all churches should follow a single rule for Easter, which should be computed independently of the Jewish calendar. But, the actual way of the computation didn’t get sorted until hundreds of years later at the Synod of Whitby in 664AD. Bede mentions in his writings the argument about the different ways of computation, which is also collaborated by Columbanus (a missionary). The Roman’s won the day, and hence forth they became the stronger, diminishing the Celtic church nearly altogether (which of course, has since been revived by certain groups in modern day). There are still arguments even today for reform of the dating of Easter.
You make a good point about Ronald Hutton’s research – Some years ago, I read ‘Blood and Mistletoe: The History of the Druids in Britain’ – it is certainly a very good book, that brings clarity to many issues – I recommend it.