(Spoliers!) Just finished Cold Days. Have some questions!

First off Wow loved it!!

I have a few questions before I move onto Skin Game, and what better place than here to ask!

So, Nemesis is an Outsider that's infecting certain people, and as far as we know it started with the knife that Bianca had, and gifted to Lea?

Is it established how it, and the other outsiders (who attack the island with Sharkface) got passed the gate that Rashid protects?

Kringle and Odin are the same person? He just assumes the mantle of Santa Claus at a certain time of year?

The apartment that Molly gets from the Svartalves do the events that lead up to that happen in a short story?

The building that Redcap blows up with everyone inside. Dresden mentions being there before. Is that also a call back to a short story outside of the main series?

Thanks in advance for the help! I haven't read any of the short stories, and so I feel like I'm missing some stuff. I'm going to pick up Side Jobs next I think to fill in the holes, and then track down the other short stories or wait and hope that a second collection with the remaining stories comes out soon too.