Ash, I think you need to slow down, especially in regards to ADK. You say [to Andrew]:
But you stated it as fact. Not, "I assume there are..." or "probably we're dealing with..." And who actually knows the number of scum in a game? Scum.
Occam's Razor points to you. The fact that ADK backed you up immediately afterward, as well as adding in his knowledge of at least one and possibly more third parties, is very bad for him. He's either your partner or a third party himself. Because the OP also doesn't guaranteed a third party, and yet he says there is at least one.
Here, you say that ADK is "adding in his knowledge of at least one...third party". You're tying this to the fact that he stated this as a fact, rather than "I assume" or "probably", but going back to look at ADK's post, we see:
There's only three scum but there's also possibly (probably) at least one third party, i.e. a serial killer, which both town and scum have to worry about.
These are the exact same reservations that you just said that wouldn't be scummy if made, it's only scummy when presented as a fact. But it's very clear ADK is putting this forward rather than presenting this as a fact.
Also, you seem to be saying that it's likely ADK is either "Andrew's partner or a third party himself". But it's impossible to pin both possibilities on him at once. If he's mafia, he has no clue if there's a third-party. If he's third-party, he has no confirmation on the number of scum. So saying he's either one of the other for a slip doesn't make sense when the two different things you are accusing him of don't have knowledge of one another.
I think there's a very good chance there's 3 scum personally. It's an assumption I make in a 13-player game. I don't recall seeing a 13-player game without 3 scum -- 2 is tough on mafia, 4 is tough on town. I don't think making that assumption and treating it like fact is truly scummy. I know that the scum team in Innovation (where I was scum) painted Xeiron (town) as scum for making the exact same mistake, when in reality, it's an assumption everyone makes. I'm sure that everyone coming in assumed 3 mafia -- I know I did. Yes, you want to make sure that you say "I am pretty sure there's 3 mafia, but I'm open to other possibilities / not getting blindsides by an extra scum", but saying "there's 3 mafia" doesn't indicate scumminess by itself, however.
What I do find scummy, though, is Andrew's reactions to all of this.