I am still (not) making waves. This is going on right now
The Gaels did not speak Germanic. Their word for Saxon is Saesanact
MJ Harper in his book The History of Britain makes for a reasonable case that at least some did.
The Belgae didn’t speak a Germanic language, as is clear from their coins and personal names. The ones in Britain, at least. I haven’t studied the Belgae on the continent in any depth but it is my understanding that the academic consensus is that they spoke a Celtic language, not Germanic. Don't know who MJ Harper is but if he claims the Gaels are Germanic, he's mistaken, likewise the Belgae & the Atrebates are Celtic. You are confusing Celts with Germans.
He does not state the proto Irish were Germanic, just that many could have been speaking 'English'.
Not likely. English didn't even exist in the 5th century. Though I personally doubt it, it's possible that a few Gaels might understand Saxon then. It's most definitely not their native tongue. Personally I doubt that even one Gael could speak Saxon in the 5th century. Too little contact with each other. I believe there is a consensus of one that believe the Belgae are Germans.
The North Sea used to be called the German Sea. Norway, Sweden, Denmark and Germany all have a Germanic language. It makes complete sense that the other areas around that sea, modern day northern Belgium (Flanders) and England also shared that language root, simply due to trade. The reason why English is the major language today is due to trade. It's not everyone's first language, but it is most peoples second.
Today & the 5th century are very different times. As I said English didn't exist yet. The Scotti were creatures of the Irish sea not the North sea. It's doubtful that a Scotti & a Saxon had ever met at this time. The Scotti are to the west of the Britons, the Saxons to the east. The Saxons have to carve up & digest the Britons before they have much contact with the Scotti, that's gonna take a few centuries. In fact the Saxons never quite complete the task before they are themselves conquered by the Normans.
Mick Harper's book is a spoof history. It's written in the vein of the Henry Root Letters. Harper's law of applied epistemology allows him to slice Occam's razor through centuries of academic assumption.
"At some point in Michael Harper’s life, a member of the academic establishment must have done something truly awful to him – like decapitating his Action Man®. "
He is taking the p**s out of the academic establishment by making claims that the English have always spoken English since England was repopulated at the end of the ice age. He loves it when a few academics take him seriously and challenge him.
There is some truth to what you say, but let's compare the apparent conquest of England by Anglo-Saxons with that of the definite invasion by the Normans. Archaeological evidence proves that 'peasants' were not pushed out, by either invader, confirmed by other sciences as well. So how were the AS invaders able to achieve the shift in language from 'Celtic' to Germanic, and what mechanism was used?
The entire "English as a fourth Germanic language" is fringe theory which was adopted by ambitious archaeologists who never held any research post in any academic institution like Francis Pryor. They latched onto the experimental work of a geneticist, Peter Forster, whose algorithm used the evolution of human biology as a proxy for the evolution of language. He had a 4000 year margin of error for the date when English began to be spoken. No linguist would countenance it, simply asking, "Where is the evidence"? There is not a shred of evidence.
It was a very short lived hypothesis, one to two years, that was never taken up as there is simply no evidence. It was posited in support of the minimalist acculturation models because that model had been unable to explain the swift and and widespread transformation from Brythonic to Anglo-Saxon. The minimalists argued that "they didn't have to because Anglo-Saxon was always spoken".
No linguist discusses it and it is never cited in peer reviewed papers. The people who jumped on this short lived bandwagon, people like archaeologist Win Scutt, were writing papers which were not published. If English had been spoken, there would of course be English place names dating to that period but the poor quality of his work undid his assertion when he was claiming that places like Meysey did prove a Germanic language was spoken in the pre-Roman period when anyone with a book on place names could have told him that it was named after the 12th century De Maisey family who came from Normandy.
It was a hypothesis created by the medical practitioner Stephen Oppenheimer who relied on the work of a geneticist, Peter Forster, to help sell his book which was based on the hypothesis that the Basques were some sort of living fossil of the European population during the Ice Age, a notion too that has been comprehensively trashed. No peer reviewed researcher ever took it up and it died a death within a couple of years. It was a good marketing campaign which sold a lot of Oppenheimer tests at £420 each plus the book sales but it was never science.