That note seemed to only be for the benefit of the viewers. If she planted it, it could have been so that the security cameras (if any) captured her pulling it from the door and reading it. But if the security cameras could capture that, it would also capture who placed the note there in the first place.
If her intention was simply to have the note so she could tell the police that she saw it pinned to the door, then there was no need to pin it to the door, or have a shocked/concerned look on her face when she read it; just have it in her pocket and say she took it off the door. But see security camera above. The show didn’t disclose if there was a security camera, if the handwriting was familiar, if there were fingerprints found on the note, etc. They glossed over that whole area, IMHO, because its only purpose was misdirection of the viewer.
As far as the tox screen on Kono’s blood, my fan theory is that she was conspiring with the Dimas’. They had their corpse jewelry thieving business to hide and the younger one’s murder of the young girl. With Oscar being released and able to do more research on his case, my theory is that they hired someone like Jan, who nobody would suspect, to kill Tim Kono while they had a solid alibi by being on the security camera.
They think it has all gone swimmingly because his death is ruled a suicide, but then our heros Martin, Short, and Gomez start doing what seems like a silly podcast until it gains traction. Then Dimas becomes their sponsor to get control of the podcast, uses his “juice” to suppress the tox screen and to get them evicted from the building.
But, they realize, that our heroes are not going to go quietly, that they pinned the murder on Jan, and are staying in the building and will continue to hamper the corpse jewelry stealing. So they murder the nasty building superintendent lady and try to pin it on Gomez (because she was there the night of the girl’s death) and get rid of her while texting Martin and Short to get them out of the building and give them an alibi, likely so they can wrap up the podcast and blackmail them further into not disclosing the jewelry stuff.
But again it doesn’t go as planned because Martin and Short get pinned for the building lady’s murder.
Just my wild fan theory.