Yes, you can complete the Sinnoh Dex in Platinum.
As for your other question, Manaphy IS tricky if you don't know where to look. In Mr. Backlots mansion, in the room with Mr backlot, there is a table, with a journal on it. If you interact with that journal, you will get the "seen" entry for Manaphy.
And if you wanted to get the "caught" entry for Manaphy, then you would use the Pokemon Ranger method of transferring it over. However, it was only available through ranger net on all 3 games, and since nintendo terminated all online stuff for the wii and ds, it's no longer obtainable.