Great Old One
If no one actually starts it then no combat. If someone declares to start it then roll initiative and enemies that go ahead that player realize his intent ie he started to move for his weapon or otherwise hostile act but they were prepared and reacted faster.
That would be generally true, but the problem remains that that preparedness is worth nothing as per the RAW, they don't get a bonus to initiative and they don't get prepared actions.
Assuming his target is otherwise distracted in the room then lockpick check to determine if he gets through silently. If he fails then target is aware. A particularly bad fail might result in the target being aware and the door staying locked.
On this one in particular, I agree that the scenario was not described in enough detail, because I did not want to railroad solutions directly to prepared actions, but the idea was to stay hidden but to ready an action for when the victim came out of the room.
Make the scene as concrete as needed. If the player suspects the assassain is behind the desk and the player carefully watches the desk when entering the room then he will not be surprised if the assassain is behind the desk. If the assassain is somewhere else or he doesn't have some specific idea of where the assassain is then perception vs stealth.
It's a good way to deal with it, on this one, my point was that, if prepared actions are allowed, the potential victim could declare a dodge if he sees an assassin emerge (from the desk, for example).
The orcs are either surprised or not surprised. 1 character cannot surprise the orcs while the others do not. This would occur vs normal stealth vs perception rules.
It's true, but not really the point in this case, it was more about coordination, on this @Umbran perfectly understood what I meant and how prepared actions to act after one ambusher triggers the rest is the right solution (at least the way I see it).
This depends on the specifics of the guards etc. He may not even be able to get close to the individual. He may be able to get close but have no chance of escape.
Very true, the question was, assuming that he is, he should be able to prepare something for his strike and getaway.