It was obviously meant as a wink and a nod to the Christmas Eve story, but it seemed kind of odd to me that at Mid-Summer, Michael was already telling Harry he was invited for Christmas.  It would have been more cohesive with something like "our home is always open to you".  Anyone else feel like that seemed a bit weird?

First, I think that is probably reading too much into it.

That being said, if there is significance in her calling him that, I think it actually would indicate the reverse--a White Council Wizard would be a capital "W" wizard, like an official title.  Wizard lowercase is just Mab calling him what he is, a wizard.  There is of course nothing keeping Mab from referring to him as a "wizard" and having it mean nothing has changed with his WC status.  I would tend to think it's her way of acknowledging he's a slight improvement over your generic "mortal".

Great topic!  Despite the Christmas Eve story indicating a huge number of deaths, I think Battleground will wind up being comparable to the Battle of Winterfell in GoT--a few key characters and lots and lots of unnamed going to their deaths.