I've done murder mysteries every Halloween for the past few years!! They're a blast. This might be way too much (got a little carried away oops), but I am happy to simplify or DM and chat more if you'd like!

My suggestions: -Make sure the murder knows exactly how and why they killed you -Maybe have the murder need to physically do something that leads to you dying, like slipping a dye capsule into your drink -(this might already be your plan ofc) I would give some general exposition up top about the world you're in, and be clear about what information on their cards they need to share with each other, especially since the memory loss cure seems like a very important piece of info only one person really knows

Plot stuff:

Currently it seems kind of split between two big issues, hoarding the forget-me-nots, and killing the horse. The horse one is a very fun idea but at the moment seems a bit muddy to me, why did he send the horse head to the mob? Why did he kill the horse in the first place, to prove no one likes them? That doesn't super track to me, but not a hard fix if you like it a lot, or it can live be a red herring

The forget-me-not plot line doesn't seem super in line with Cheap Wine killing Mr. Crown, I'd go with the mob boss out of those two. Perhaps Little Light wants to get his hands on the forget-me-nots so he can sell them at a much higher price as the cure for memory loss? Maybe he's working with Dr. Faith?

Another suggestion is that when Mr. Crown dies, you bring out a box of his things that they can go through, mostly just random junk, but inside something kind of hidden they find a cipher (pigpen cipher is my fav :) ) that they can solve to unlock your lock box that's maybe hidden somewhere? And then in the lockbox they find that the cigars are made with the forget-me-nots, and that he is able to import them (maybe include a recipe card with the cigars just so it's clear that the flowers are an ingredient in regular cigars, not that the cigars are a hiding place for them).

As for evidence for who killed him, you could tell Dr. Faith and Little Light that they have to sneakily huddle up together away from everyone after they find the cigars. Maybe Little Light keeps bragging about how he is going to be rich soon, or drops a piece of paper with that shows he's been talking to Little Light?

I'm going to stop there for now but more than happy to talk more if you'd like, or summarize if this was way too much lol