You cannot breed legendaries.
No.
You can not get a Turtwig egg in Pokemon Platinum. But if you breed a Turtwig with a Ditto, then it is possible.
Nothing. Since a Ditto can no breed with another Ditto. They will produce no egg.
Yes you always can get an egg if you breed with ditto.
No
no you cant
No, the egg will not be shiny. However, it is possible that the egg will hatch into a shiny Pokemon, but the chances of getting a shiny Pokemon from the egg are not any higher than they would be if the ditto were not shiny.
You can find a Ditto egg in the Lab, in the Shelter, in the Egg Supplier. Sadly, Dittos can't be bred.
Yes, Feraligatr can breed and lay an Egg with a Ditto however it'll be a Totodile Egg.
The only Legendaries Ditto can breed with are Manaphy and Phione, which its egg will hatch a Phione. All other Legendary Pokemon cannot breed.
Yes, any of Eevee's evolutions and a ditto can be bred for an Eevee egg.
A Dragonair and a Ditto will have a Dratini egg.