Last Seen sounds like an ambient track to be lost to, and doesn't really set the tone of what you've described in the text: a central musical theme for a game with a heavy tone like trying to track down a killer. The track feels considerably more lighthearted -- still serious, but not on the side of I am trying to find justice for something very wrong and horrible that happened to us.
Looking Into It is very contemplative, and sets a good tone for any investigative work. It may express frustration at not getting closer (the guitar is really good at communicating that), but it can also communicate the gravity of a given situation. There is no room for error and this case is getting cold if we don't do a thing about it, or maybe it is already cold and we're fighting against all hope. Even when the synths and drums kick in, it does convey that desperate fight -- desperate isn't the right word. It's more, hope against all hope. When your C#minor -> F#minor bridge comes in, I still find it a touch too lighthearted for the subject matter. When it tones back down the gravitas is back.
Then THAT Means... I sense some very strong Ace Attorney influence. The bass and opening synth do give that sense of concentration and urgency in solving logic puzzles. The key shifts are sublime here, and they do convey the idea of cracking one's head to solve things, an air of mystery. This is your strongest track.
Kelsey. Now this is also an excellent cue to describe a massive plot twist where she is unearthed as the killer. It's slow, it's measured, it's methodical, the detune adds a level of chaos and confusion and extreme emotions associated with this scene.
Unhide got my chest pounding hard. It is a wonderfully tense, your life depends on this escape sequence with a very singable and memorable melody. I do put it up there in this OST, a very close second to Then THAT Means... . The jazz tonality works really well here and also throws me right back to noir films and thrillers where this music is front and centre. (Probably the thing that would seal the deal would be subtler, greater with finesse, but equally urgent, jazz drums.)
I have no complaints with the mixing.
This is an OST that does take a while to come into its own, but oh when it does, it really, TRULY does. There's a lot you've done in here that is simply delightful, and as such you should be proud. All the best in this jam!