1. http://projectpokemon.org/forums/showthread.php?16601-NDS-How-to-Backup-Restore-your-retail-cartridge-save-file-for-DS-games
2. It would be easier to use the pk2pk tool Kaphotics made, located here: http://projectpokemon.org/forums/showthread.php?41681-Time-Capsule-Transfer-Tool-(-pk2pk-) . Just use it to convert the 3rd gen pokemon to 4th gen pokemon, then put those in the Platinum save file.
3. Don't know about that one.