After you got the national Dex and caught/fought heatran you go back to spear pillar and you'll see a blue diamond and a pink diamond (blue-dialga, pink-palkia)
It sure is
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no
No you cannot. You can only catch Dialga in Pokemon diamond and platinum. Although you can only catch palkia in pearl and platinum.
No.
After Palkia you get Dialga
yes
Beat the Pokemon league again and go back to spear pillar and you will see dialga again.
Trade with Pokemon Pearl or Pokemon Platinum
giritina, palkia and dialga
it's impossible, and I have action replay
Spear pillar