The only way you can see all the Pokemon on diamond is by going into the GTS and global trade different Pokemon.
There are 493 Pokemon
yes but it has to be 500 dullers
There are 493 and I am not going to list them all here!
Give them to your Arceus, if you have one, or a Pokemon to boost a specific move's type, Or you can sell them for 500 each.
by going all over sinnoh and by trade
By seeing all the Pokemon in sinnoh and going to rowen's lab.
No. not any time soon, if at all
In one of the Cities there is going to be a transfer machine, i think and it will give you all your old Pokemon
Technically, no, but every pokemon has different stats, and it is possible to catch 500 different pokemon and all of the females have less Hp than the males.
You can get Dive in Pokemon Emerald by going to Mossdeep City and ensure you check all the houses.
There is no level 500 Pokemon. The highest level that a Pokemon can be is level 100.