no because Route 16 is on Pokemon leafgreen and firered
Go to Spear Pillar in Mt.Coronet. It will only take 20 minutes to get there. You will vs Galactics along the way but if you've got at least level 46 Pokemon then you will be fine. Go on Youtube and type " How to go to Spear Pillar on Diamond" if you don't know how to get to spear pillar. Tip: Start from Hearthome and go far down and keep going far left into Mt.Coronet.
REGIS
Palkia is NOT obtainable in Pokemon Fire Red/Leaf Green, it is a generation IV Pokemon first available in the Pokemon diamond/pearl games as well as platinum and will be obtainable in Heart Gold/Soul Silver
by going going in the safari zone
The distortion world, you get there by going to spear pillar after going through Cyrus' base
The only way you can see all the Pokemon on diamond is by going into the GTS and global trade different Pokemon.
by going to the spear pillar after you get the national pokedex
Going by the national dex it's Prinplup
there is no region after sinnoh in Pokemon diamond and Pokemon pearl.
Rival
You have to get every single Pokemon their is. Then you should go to the spear pillar. Then there will be Pokemon selector. You will have to use your strongest legendary Pokemon. Then you will battle exact clones of your Pokemon. You will have to win. ONce you win there will be a portal right in front of you. Then you have gotten into the reverse world.By going in the reverse world all your Pokemon will be level one million and they will also be shiny.
In Pokémon Diamond, the man that is blocking your access to Sunyshore City will not move until you go through Mt. Coronet in order to make your way to Spear Pillar so that you will need to deal with the Team Galactic situation as well as Dialga. Additionally you have to go into Team Galactic's Headquarters in Veilstone City before going to Spear Pillar.