That's the thing. It doesn't matter whether a creature is perceived as a threat. It matters whether the creature is perceived at all. It's a forgone conclusion when determining surprise that the creature is a threat because you are already in combat with it. I understand the you are using a houserule and am just expressing my distaste for the rules-interpretation it seems to rely on.
The bolded part is where you are factually incorrect per 5E rules.
You even quote the rule yourself! "A character or monster who doesn't notice
a threat is surprised at the start of the encounter."
Remember, this is on p189, the first page of the chapter that tells us how combat works: in combat rounds in initiative order having determined surprise.
The 'start of the encounter' here refers to the start of combat rounds. The surprise rules are all about how to modify what creatures can do/not do
in the first combat round, not about roleplaying their astonishment.
What Plaguescarred thinks is a variant rule is actually RAW. In the
usual D&D-type encounter, you see orcs and you believe that orcs are a threat and so you
chaaaarge!...and that's where the combat and surprise rules come in.
But when you are in diplomatic talks, as soon as the Cormyrian ambassador walks into the room with his entourage, if you shout "Chaaaaarge!"....then I think you might have missed a day of diplomat training...!
This diplomatic negotiation is not a combat encounter. The other diplomats are not 'threats' in combat terms, so seeing the diplomats is not 'noticing a threat'. Now, when one of them gets a homicidal twitch in his eye, draws a dagger and stabs you (or tries to)....
that's a threat you can notice!
And to notice
that kind of threat, Perception versus Stealth is inappropriate, because you've noticed his
presence half an hour ago! But he wasn't a
threat until he went for his dagger with murderous intent.
And to notice
this threat, Deception versus Insight is appropriate. Just like the gunfighter, it's his body language that may give him away, and your skill at reading people (Insight) that lets you notice that he has murderous intent.
You see, 'surprise' and 'initiative' are measuring different things. Initiative measures reaction speed, but that only matters once you have started to react! It's like the starter pistol to start the 100m sprint. The reaction speed only begins when the starter pistol fires.
The moment you notice a threat is when the starter pistol fires for you. Your initiative measures how fast you react to a stimulus, but it's noticing a threat that sets that reaction off.
So you're an hour into the negotiations when suddenly one of the diplomats twitches maniacally and draws his dagger. Do you react in time? It depends. Did you think he was going to sneeze, at it came as a shock to you when he stabbed someone? Or did you read him like a book and your fine-tuned reactions kick in while the rest of them were standing about like sheep? That's what the Deception versus Insight contest is for. It doesn't matter how quick your reaction time is if you don't even start reacting because you didn't realise that there is a threat to react
to!