I vote for Bikkit as prime suspect
1. Suspicious footprints in the cookie
2. The fact that Mr Pin is described as "young". Bikkit is the youngest here, and I would never have called Mr Pin "young."
Why was the pestle taken?
What can you grind with a pestle?
Why was it made of marble?
Was the pestle the murder weapon?
And, the killers are working for a secret client. Who is this client and is the client important?
If the killers were knocking off Guild leaders, why did they kill Jan?
Was the reason they killed Jan, because they wanted the pestle, not because they wanted her out of the way?