Another of frank353's questions was about 'We are so not having this conversation' -- that's similar to 'so was von' for emphasis in German. I think the meaning is probably future: 'I am absolutely not going to have this conversation with you (right now).'
I agree that 'Don't "Lord Grantham" me' could have been an anachronism for the 1910s, though I suppose the only way to tell for sure would be to find it somewhere in the OED. Who knows, that last line in #8 might even have been more plausible as 'I will not, however, be he.'
As far as I know, Julian Fellowes is an entertainer, not a historian, and while he may indeed be careful, no one is infallible. The second season of Downton Abbey even bordered on the soap-opera-ish as far as the plot was concerned (albeit still with gorgeous costumes and settings, and good actors), and the language struck me as largely modern. They may have assumed they couldn't sell it if they demanded too much of modern (particularly American) viewers. Sort of the same mindset that led publishers to 'translate' Harry Potter for American audiences. I'm glad Downton Abbey at least chose a mostly mid-Atlantic middle ground, so at least it doesn't jar in the other direction, with Britons talking about sweaters or elevators or whatever. But the overall effect may have been blander than it had to be, a language somewhat unnaturally stripped of time and place.