Titus Makin Jr., who plays Tim Cabot, starred with Nathan Fillion on his new show The Rookie (2018).
The victim's name is Edmund Smith. It's a tip of the hat to Edmund Gwenn, who played Santa Claus in 1947's Miracle on 34th Street (1947). A second character is named Gwenn Harwin.
When Castle is telling Beckett about his Christmas decorations at the loft, he comments that he has his garlands shipped in from near the Canadian border. Nathan Fillion and Stana Katic are both native Canadians, with Katic coming from Ontario, which is just across the border from New York.
At the end of the episode, when Beckett unexpectedly shows up at the loft, she tells Castle that she got Detective Karpowski to cover for her. Karpowski previously appeared in two episodes of season two, played by Diana Maria Riva, and up until this episode had not been seen or mentioned since.
Steven Eckholdt who is Michael Case, the owner of the company where the deceased Santa did his last gig, also played Mark in Friends - he was the one who Ross thought Rachel cheated on him with.