I've been working on this in a notepad file that was last updated this morning, and I'm at the end of my rope as far as what I can spin without knowing further. Can't wait to see where I was wrong.
Secret Santa Detective File
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Peng - MHD, unquestionably
MHD - Well of course I know him, he's me!
Goodnight_a - Got a drawing and a review, which contains CYS Discord Shibboleths. Pointedly aware of Goodnight's general site activity in a distinctly Miz-like way. Mizal did imply she was going to draw a thing, I think we got 'em. ADDENDUM- I did indeed find out this was Mizal.
Darius - Feels like ogre did this based purely on vibe and the fact that none of the other gifts seem ogrey
TCat - Darius, unquestionably
Zake - Analogue pencil drawing done by someone with an unlined notebook and a straightedge, these seem like things somebody in school would have, but they aren't exactly school equipment so it could be an adult who draws occassionally. Would not be Miz or Ogre. Suranna draws but electrically. Would not be Darius or MHD.
Ogre - Whoever this is probably uses the discord because being a chef wasn't one of the known traits of ogre before all this.
CavusRex - Given a review. Mizal, Suranna, & Peng, already commented so their odds are significantly lowered. Mizal wrote an *actual* review the first time, so it would be weird to do it twice. Zake would do this but it feels like it doesn't seem to follow a Zake structure. Too lighthearted to be TCat. Possibly it was Gryphon or Sherboid. I think this was Sherb for some reason.
Suranna - A borderline Suranna-negative poem, giving vibes of Mizal or Petros. Leaning on the side of Mizal because the poem is about cracking the productivity whip and Petros probably would've just drawn the picture if he had it in him. ADDENDUM: It's gotta be Petros.
Gryphon - A poem. Feels like TCat, Suranna, or Stargirl made this by the way that it's bent around the rhymes.
Stargirl - electronic drawing, seemingly made off of a model. Zake does these, but so does Suranna. Leaning on the side of Zake because Suranna is more freeform and uses far thicker/more variable lines, probably a stylus.
DBNB - N/A
Tim - Given a shitpost poem image that could be made with very common computer tools. Not a single shibboleth except Mogami River, which was already spoiled in the OP. Someone very young who doesn't know Tim may have done this. Goodnight_a or Stargirl are in the age range. Or maybe not, they've been here forever but my mind still sees babies when I look at their usernames.
Petros - Looks like suranna could've done it, but Sherbet thinks this is TCat and some of the review does read as Tcattish. Theorizing that this is TCat.
Mizal - N/A
Sentinel - a fuckin DIGITAL *PAINTING!?* Completely out of left field, can't think of anyone who does this. Regardless both included reviews of randomly walk so I have to kill them both whoever they are.
Sherbet - N/A