no
get nationaldex then every day talk to Mr back lot until he sez he saw a Evee in the trophy garden
No, you cannot get Arceus with or without completing the game. The only way to get Arceus is by a Nintendo Event involving Mystery gift and the Azure Flute.
beat the eliete 4 with 60 Pokemon owned get it from oak or talk to oak with 60 Pokemon
You can't. You have to get NationalDex and then talk to Bebe. She will give you a free eevee, though I think it only works once
You cannot get a Mewtwo in Platinum.
you cant get mudkip without trading on platinum
No, you cannot catch a mew in Pokemon Platinum without cheating.
There is no way to get Bulbasaur in Platinum without trading or Pal Parking.
Not without cheating
The only way to get Shaymin in Platinum without cheating or an event is through trading it with a friend.
You gain nothing except the satisfaction of completing the longest Pokedex to date. Talk to Professor Oak after you have completed your Pokedex (which to this day is still not possible without the use of a cheating device.) and he will give you a Up-Grade which is useless now because chances are if you have a complete pokedex you already had a evolved porygon. He will than praise you for completing the task.
you can't without a cheat