Nice theories, but I'm not sure if I'm that bowled over by them. But... That's the joy of writing historical fiction. You're free to use a pick n' mix approach to the sources!

I still think the frontier was moved north from the line of Hadrian's Wall to Antonine's Wall (remember, the Gask Ridge can also be seen as some kind of informal frontier) is more to do with Roman politics than any real or perceived threats from the Roman barbarians.