Really excellent detective game, impressive work. If the full game can match this level of detail I think you have something really special on your hands.
I particularly liked that every logical followup I wanted to ask was asked right away.
Small things that could be improved:
I found it annoying to keep opening up the dialogue topic selector and selecting the same thing over and over just to exhaust all the dialogue options within the topic. I can see why making this cumbersome would be intentional, but players are gonna want every detail instead of focusing on deduction first, so might as well streamline by going back to the topic dialogue options instead of back to the main topic selector.
Double the walk speed please, it takes 10-20 seconds to get anywhere which adds up a lot walking back and forth. On the one hand I like having time to think and contemplate (and I really love how quiet the game's soundscape is, calming and serene, but also unnerving), on the other hand I don't want to walk to the end of an empty garden patch and have to walk all the way back when I got nothing out of it.
I think the art is great, it's a really cohesive and astute observation of this style of game I haven't seen done before. The faces have lots of character and personality. The ui matches and it's really easy to understand and pick up. I would recommend making the dialogue text typewrite from the left to right instead of being center-aligned, it'd be easier on the eyes, because right now I always have to click to skip it so I can actually read it. The typewriting is so slow you would expect to be able to read it as it happens, but since it shifts with every single character your eyes can't get a fix on it (this is a common ux mistake with dialogue text).
I wish I could've solved a scene in the demo, that would have capped it off better I think. Let the player see the full gameplay loop before letting them go. You mentioned in a comment that it is possible to solve one scene, but I scoured the entire grounds, asked about everything to everyone, and nothing turned red in my notebook to allow me to solve anything.
I found it strange that the metal pipe on the path that matched the model of the metal pipe behind the shed couldn't be interacted with. Since the game is so low-fidelity I guess you have to be careful with reusing models like that. I also wish I could've asked the gardener if I could have access to the shed. It also felt strange that I could only ask people about the drag marks, but clearly it was caused by the barrel, but I couldn't ask about the barrel or connect it to the barrel in any way.
I found the flower petals in her apartment, when I ask her about the flower petals at the grounds it should have remembered that and let me ask about it instead of her just ignoring it. I also wish the newspaper on the bench that I could connect Wilkes' name to her neighbor's name and ask her about it (grabbing words from notes and text in the world could be cool). I'm assuming her neighbor is a relative of the governor and she knew something that she shouldn't have (or she just witnessed a murder happening that she shouldn't have, which is more plausible from what the demo showed so far).
I wish your partner wasn't such a dull dud of a person. I don't get why he's a PI (I guess we don't actually know if they're detectives?).
Overall, really impressive detective game. This genre is incredibly hard to pull off but I think you have a really great system so far and it seems like you have a clear vision for where the case is gonna go with lots and lots and lots of complexity.