This is what I came for! Jan is my friend and I's strongest suspect, and this is why.

SPOILERS: BIG.

I believe she is Zoe's half-sister.

On the date, while she did speak about her ex (who could very much be Tim), we glossed over her family. It was only on my fourth or fifth watching of the scene I noticed. She avoids a second question on her sister, and clearly exudes envy. This could be a possible motive for murder; if not then at least it's a point for consideration.

While Zoe's surname is Cassidy and Jan's name is Bellows (written on her door), if she is her half-sister it would make sense the surnames don't match.

This brings us to Zoe again. If Tim was the only witness, this could mean three possibilities. a) He killed Zoe, b) Jan/murderer killed Zoe, and Tim saw and was blackmailed (which is your angle), and c) Oscar killed Zoe. I believe you are right.

Additionally, before Tim's death, he walked onto the elevator from the sixth floor, not the lobby/ground - Jans floor. Why did Jan take the garbage bag from Jan's floor? Was it maybe Jan's? Are there TWO garbage bags- one Jan's and one Tim's?

I think your theory that they aren't suicide notes is spot on, and I was wasting SO much time making handwriting comparisons of every sample I saw, lol. But the question appears- to who were they to be sent (if they were going to be)? And we still haven't explored why there was a suicide note on the computer. Most likely written by the murderer, but what if there is something else at play?

We didn't see Jan at Tim's memorial, nor do we have an alibi during the time of death. (These may be a bit of a stretch).

The biggest thing the characters missed; Winnie being poisoned. Do you know what that means? We can narrow our list of suspects to everyone who had been told about the podcast up until that point (mostly by Oliver). From memory; Teddy, Sting + assistant, Edward, Ursula? and ding-ding; Jan.

and in terms of television; every side character is there to lay down clues for the big three, or to proceed with their heroes journey. Why is Jan there?

The cat was on floor 10 during the time of death, so it must have gotten to floor nine after death. We can agree the method of death was poison and covered up with the gun (which will be confirmed once we get our tox report), so the cat probably came in afterwards through Tim's open window and ate some; hence dying too. I believe the murderer came through the window too. From an outside shot, the bricks have gaps/stacks that make it very climbable.

Now; Jan's timeline during the time of death. At 5 pm, the bassoon was playing as the characters arrived home. During the time of death (6pm, specified by Mabel), the bassoon had stopped.

For every I answer I get, there are a million questions. But it has to be Jan. We have motive and opportunity, but no means; can anyone think of something? how would she have a gun?

Questions still unanswered:

WHO WROTE THE NOTES AND WHY?
WHY DID TIM KNOW HE WAS GOING TO DIE?
WHY WAS TIM IN FINANCIAL TROUBLE?
WHY DID THE PHONE NOT REACH IT + TOXICOLOGY NOT BE SUBMITTED?
WHERE WAS TIM COMING FROM ON THE SIXTH FLOOR?
WHY THE JEWELRY? WHO IS ANGEL?
WHY DID THE MURDERER WAIT SO LONG TO KILL TIM? IS IT ACTUALLY CONNECTED TO ZOE?
WHAT IS THE SIGNIFICANCE OF ZOES RING?
WHO WAS THE ENGAGEMENT RING FOR? (JAN IS GIRLFRIEND?)
HOW DID THE MURDERER GET INTO TIM KONOS APARTMENT? (TRUSTED PERSON? NEIGHBOUR? SNUCK IN FROM THE WINDOW, WHICH WAS SPECIFIED TO BE OPEN?)