No
It is impossible to change Ditto's form outside of battle. It only changes by using the move transform during a battle.
Dive is not an HM move in Pokemon Platinum. There are, however, Pokemon that can learn it by level up.
Dive and other moves like whirlpool and headbutt cannot be used outside of battle in Sinnoh. Different games will have different moves that can be used outside of battle, such as in Hoenn, secret power can make a base, or in Johto, Headbutt will smack a tree, etc.
Dive is absent in Pokemon pearl,diamond,platinum,heartgold,and soulsilver . But will return in Black/White =-)
Dive is not an HM available in Leafgreen. If you want to teach a pokemon Dive trade it to any of the Hoenn Region pokemon games (Ruby, Sapphire, Emerald) and use the HM Dive to teach the move and then you can trade it back. NOTE you cannot use Dive outside of battle just like Ruby, Sapphire, Emerald since it's not an HM instead it's a regular move that can be removed either by the Move Deleter or by teaching a different move.
No. Dive is an HM in kanto, so you can't use it out of battle. Some Pokemon can learn dive with no Tms or HMs, but you can't use it as a field move.
there is a attack dive which my wailord learned in level 46
The use of the move "dig" outside of battle is that it can get you out of caves and things like caves without using an Escape Rope.
You can use the move sweet scent.
When using Surf outside of battle, and if there is a waterfall and you are at the bottom, you can travel up the waterfall by using Waterfall.
If you are talking about Defog(HM05), then you will have to beat Crasher Wake first in order to use it outside of battle. It is somewhere in the Great Marsh. In Platinum, it's Fantina, and after you beat her, you can use Surf.
Dive is not a hidden move in leafgreen.