Thought about Artie/Mike plot
I've wondered if the intent of the Artie/Mike plot was that Artie was what Mike would become if he didn't get over his jealousy ... that he was in essence the "spirit of Christmas yet to be" for Mike. Does that make sense? To me, that ties up part of the plot issues others have had (how Artie found Mike's apartment, how he knows Mike's favorite cookies, the glass angel replacing the onen Mike broke when he knocks the guy into the Christmas tree) It doesn't tie together seeing Mike as his ex-wife though ....
Just throwing it out there as another possibility - it worked for me to show why Mike could convincingly change his jealous behavior if he believes that it could end up killing his soon-to-be wife.