the fact that he calls Karen 'Katie' is quite enough to grate

Oh, I liked that! I thought the new name - together with the SHOCK AND OUTRAGE on the part of (some of) the Marlows when they heard it - was a really economical way to show how there's more to Karen than 'Kay'... and how her family are unable to accept that. We never hear Karen objecting to the name, do we?

I like Edwin, but will have to have a think before I can explain why.