First you have to complete your first one (sinnoh dex) by seeing all the Pokemon in sinnoh.
Next go to professor Rowans house and show him your pokedex.Then professor Oak will come and after a long chat he will give you the national pokedex.
There is no other pokedex.
No, there is not a Pokedex after the National Pokedex in Pokemon Diamond Version.
diamond pokedex number 80 is Barboach
In diamond is TENTACOOL but I am not sure about in other Pokemon games. In diamond is TENTACOOL but I am not sure about in other Pokemon games.
You can get other fossils when you defeat the Elite Four and the Champion and receive the National Pokedex in Pokemon Diamond.
Riolu
Oddish is the 43rd Pokemon.
in the national pokedex it is mackoke. if it is in the sinnoh pokedex it is buneary.
Number 33 is Golem on the Sinnoh Pokedex.
arcanine
Yes.
In the Sinnoh Pokedex Number 114 is Mantine. In the National Pokedex its Kabutops.