Mulaney was evidently at the Creative Arts Emmys on Saturday supporting the show, although that doesn't necessarily mean he's returning this year to write for the show.
I forgot about the 3 most recent people hired (Chris Kelly, Zach Kanin, and Neil Casey), so there's actually about 13 people on staff right now. Here's a rundown of how long they've been with the show:
'12-'13- Neil Casey
'11-'12- Zach Kanin, Chris Kelly
'10'-'11- Sarah Schneider (She joined late in the season)
'09-'10- Michael Patrick O'Brien
'07-'08- Rob Klein, Kent Sublette
'06-'07- Marika Sawyer, John Solomon
'05-'06- Colin Jost, Bryan Tucker
'00-'01- James Anderson, Erik Kenward
Plus the aforementioned Baze, Meyers, Higgins, Downey, and Lorne (who I doubt writes anything).
Four new people hired in the last 3 seasons isn't that many. You'd think they'd be trying to infuse the show with some new blood, writing-wise. James Anderson has been responsible for some of the worst stuff on the show in recent years, like "Gays in Space", "Gemini's Twin", "Deep House Dish", recently "The Californians" with Fred. John Solomon wrote a lot with Will Forte and recently was working on the Digital Shorts, so it doesn't seem like he will be around much longer.
Marika Sawyer was credited as a "writing supervisor" in this episode, so I don't know if that is maybe a promotion for her, with Jost now being co-head writer with Seth.