No, Amity Square is the only place in Pokemon Diamond, Pearl, and Platinum where you can stroll with Pokemon.
Rival
No.
You can transfer it from an old Pokemon game (such as Pokemon fire red) or trade other people.
by going going in the safari zone
The only way you can see all the Pokemon on diamond is by going into the GTS and global trade different Pokemon.
In the options it can be changed so that 'Shift' allows to swap out a Pokemon and 'Set' means the battle keeps on going.
You can clone using gts. I'm not going to write how to, but there got to be another question on how to.
there is no region after sinnoh in Pokemon diamond and Pokemon pearl.
Going by the national dex it's Prinplup
mesprit,uxie,azelf,dialga,rotom only in Pokemon diamond you get cresselia and darkrai from a Nintendo event dratini,dragonair,dragonite is not a legendary giratina you can only get it in platinum
sorry but I'm afraid arceus is impossible to get without going to a Nintendo event, unless you cheat or trade with someone who has.