no
you can't catch it because you already beaten the legendary
You can catch Dialga, Uxie, Azelf, and Mesprit
In Pokemon diamond you can catch Dialga, Azelf, Uxie, Mesprit, Cresselia and Arceus
you can start over
5
Yes quite a lot of them
no
Of course, a Master Ball catches any Pokemon without fail, Legendary or not.
there is no best spot
catch legendary Pokemon like giratina and heatran
You'll have to have received it from a special event.
uxie azelf dialga, mespirite, girantina and heetran