So you can move the adverb around, or it's just 'byth' that is unusally mobile? :) It seems to replace 'ddim'. And no SM on 'byth' here.
"Byth" as a word is resistant to mutation in some situations which is why it hasn't here. I'm not sure what you're asking here since "Byth" is in the position you would expect it to be in here.