Just watched this last night.
From what I could see, the kill order associated with "Excommunicado" went out right then, but there was an hour timer on it before it became active.
The scene in the park where everyone's phones are getting texts, and they are all looking at John, is to show that everyone is against him now and they are all just waiting for that hour to be up.
This is why even though John is badly hurt, once he sees the texts coming in he starts running. He knows he only has an hour before it's free game on his life, so he has to do what he can to be prepared when that hour is up.
As to your second question as to what the mercenaries were all doing there, that is not really explained. It seems to be a continuation of the movies theme that these people are everywhere. It could be that some where there from the the earlier part where Winston causes everyone in the park to freeze (showing his control and power), and they were in the park already for that demonstration.
EDIT: Something extra I have found that might help, not sure on the validity of the sources since they are just entertainment outlets, but they seem okay.
From an interview with the Director http://www.empireonline.com/movies/features/john-wick-2-chad-stahelski/
Despite that final scene, not everyone is an assassin
"Less than 0.5% of the population [are assassins], I would hope! It
was probably everybody in that world in Central Park. John Wick just
killed more than a hundred people, and [Ian McShane’s character]
Winston was just laying down. But the people in the final shot in the
parks – they're not all assassins. You just don't know. I like
ambiguity. It's like that Invasion Of The Body Snatchers thing – you
just don't know."
From this it does sound to me like they deliberately brought all of the assassins into the park in order to project power against John. From an interview with Keanu, it seems part of the reason for that was to set up the dangers to the character for the 3rd movie.
Hope that helps.