To finish the sinnoh dex (Pokemon platinum's,diamond, and pearl dex) you just need to see every Pokemon not catch every Pokemon.
You have see to many different pokemons. Then you will fill it (you have to every Pokemon to get national pokedex).
on platinum, you see Manaphy in Mr.backlots Pokemon mansion
Route 225 is not a route that can be found in Pokemon Firered, route 225 is in Pokemon Diamond, Pearl, and Platinum. To get there is Diamond Pearl and Platinum, you have to see every Pokemon in Sinnoh, then talk to Prof. Rowan and get the national dex.
in an egg
You see every single Pokemon, Google this "How to see every Pokemon in platinum DS"
To finish the sinnoh dex (Pokemon platinum's,diamond, and pearl dex) you just need to see every Pokemon not catch every Pokemon.
You have see to many different pokemons. Then you will fill it (you have to every Pokemon to get national pokedex).
you get a national dex so you can see even more
You don't have to get any Pokemon to get the national pokedex in platinum, but you do have to see every Pokemon in sinnoh. Don't worry, if you're thinking "Oh no! How the heck am I gonna see all those Pokemon?!", you shouldn't. If you battled as much as you could, you saw every Pokemon in sinnoh, even if you weren't a fan of tall grass.
no
on platinum, you see Manaphy in Mr.backlots Pokemon mansion
You can't see him in platinum. You have to transfer it from Pokemon Ranger
You can get Pokemon from the older versions on Pokemon Platinum, but I'm not sure if it includes every Pokemon from every game. You'll need to finish your Pokedex, then get the National Dex.
Route 225 is not a route that can be found in Pokemon Firered, route 225 is in Pokemon Diamond, Pearl, and Platinum. To get there is Diamond Pearl and Platinum, you have to see every Pokemon in Sinnoh, then talk to Prof. Rowan and get the national dex.
every pokemon from that game and generations before
no