Neckbrace. Most of it comes down to the Thanksgiving episode. Gigi uses a very strange set of words to describe cutting the quail, and Neckbrace later says something similar back at Kappa around the table (although not identical), which clued me in there might be something here.
Gigi: "Just wouldn't be the same without the whir of a motor cutting through flesh."
Hester: "This one is definitely sharpened to glide easily through roasted flesh."
The most important thing here is the timing of events at the Thanksgiving table, along with who had an alibi in the morning. Chanel and Hester get back at the house at the same time, but Hester is gone looking for a sharp knife for quite a while. Chanel #3 and 5 are at the table with #1, as are Dean Munsch and Pete. Dean Munsch exits to "freshen up" briefly, while Zayday comes in with the candle lighter. Grace and her father arrive at the same time. Chad also shows up at the very end.
Based on the timing of the above, I can deduce the following about who swapped Gigi's head for the turkey:
1. It incredibly unlikely it was #1, #3, or #5, or Pete. Although #3 and #5 bring the "bird" in, they both comment on how light it is. It would also require a conspiracy between #3 and #5, so they're both out.
2. Chad arrived far too late to engineer the swap, and he would have been noticed by someone else.
3. Grace and her dad arrive in the dining room at the same time. They were also talking about how he fathered Boone in another room the entire time they were absent.
4. Although Zayday wasn't there, she did find a candle lighter so her alibi seems passable... she's not at the top of the list of red devil suspects anyways.
This leaves Hester and Dean Munsch. Dean Munsch is absent for perhaps a minute to "freshen up." Even accounting for the fact time is probably accelerated, she took maybe 3-5 minutes tops. If I had to guess, Hester has been absent from the Thanksgiving scene for at least twice as long as Dean Munsch. So she's the most likely candidate. Hester gives Chanel and Chad a very interesting glance right as the "bird" is brought into the room. Also, Gigi had said earlier the killer needed to establish their alibi by doing "a bunch of stuff" later in the day. Where was Hester most of the day? At the Radwell's. But she didn't arrive until midway through dinner.
Now you might be saying to yourself... well, a lot of people were gone in the morning. True. But Grace was with Zayday, number #3 met up with Dean Munsch pretty quickly after returning from her family, #1 was with Chad, and Grace's father was making dinner by his lonesome self. So that leaves Pete, Chanel #5, and Hester as not having convincing morning alibis. But only Hester was absent from the table for any significant amount of time (that was onscreen). Barring some serious conspiracy level of cooperation, I think Hester is likely the last remaining red devil.
The main thing that throws a wrench in this is that a red devil was later seen at the mall, and Hester was present. I think it was very likely a fake red devil, aka Pete was sent there to scare them to get them to come back to the house (likely be Dean Munsch). I suspect Munsch might have something she's blackmailing Pete about. Pete wasn't expecting it would escalate into him murdering the policeman there with Denise, which is why he's freaked out and tells Grace he is a murderer.
Hester also gives a thumbs up in the opening sequence when the red devil appears on screen with her. Now that whole opening thing is campy and/or satirical, but it also might be a blatant give away.
And just in case I'm wrong about Hester my backups are Grace and Pete, in that order.
