Which is not to say that screwing up your thous is acceptable. *g*
For the Norse stuff, I use a different cheat. Which is to say, I want to present a Norse flavor on a different planet, because both A Companion to Wolves and the Edda of Burdens are second world fantasy. Which means they're living a culture and speaking a language that may be superficially Norse-like, but isn't historically Norse, and I handle this (having neither the inclination nor the ability to remember a language wholesale, as it were) by cherrypicking words and names from Anglo-Saxon, Norse, and Icelandic. And sometimes German, too.
We love our Sweet's Anglo-Saxon Primer. Yes, we do.
It's a balancing act. It's impossible to get everything right in any book--or frankly, even a short story. (Get back to me on stage makeup in the Elizabethan Theatre: nobody knows what they did, but there's textual evidence that they did something, so frankly, I looked at the available evidence and I made it up! (hold your nose and jump!!) and somebody, somewhere is going to disagree with me. But we have Rumour painted in tongues and we have some suggestive language in Stubbes...)
But enough about me: what Ms. Browne is suggesting in her articles as the right way to handle this stuff... pretty much is. As in most things in writing, the hard way is generally the good one. They're quite marvelous resources.
If you want to get even more in depth than these articles, I can recommend C.M. Millward's A Biography of the English Language as a starting point. Shiny book. Fascinating stuff.