In diamond are all the legendary pokemon in it?
No, you can't find all the legendaries in Diamond. There isn't a Pokemon game with all the legendaries in it. You can capture 8 legendaries in Diamond, f you want all the legendaries, you'll need to visit events, trade or migrate them.
How do you get past the person who wont let you into the road that leads to sunnyshorecity in Pokemon diamond?
There are quite a few. Uxi/UxieMespritAzelfDialgaPalkia (Unobtainable but u can get it's info in your pokedex)HeatranRotomShayminManaphy (Not sure if its a legendary)Phione (Not sure either)DarkraiCresseliaArceus And i think that's it... the LEGALLY UNOBTAINABLE ONES IN POKEMON DIAMOND: ShayminManaphyPhioneDarkraiArceus