The shockwave reached the GDrax node in under 15 minutes. Why?
Play a mission which takes place there, near the GDrax node in Capella.
High Noon, for example. Now take a good look at the star. Note that it looks suspiciously much like our sun. That means you - and therefore the GDrax node - at about the same distance from Capella as the Earth is from the sun, 8 light-minutes. Say 5 to 15 light-minutes, to have an acceptable margin.
That means that the Shivans would have had only 5 to 15 minutes to evacuate.
Now play
Into the Lion's Den. Count the juggernauts jumping in (about 5, IIRC). That number could have left Capella in time. The others would have been destroyed.
As for the intrasystem jump theory, play through the main campaign again. Write down any mission which does not take place at about the same distance from the star. None.
Subspace seems to be limited to a thin sphere, and most likely even a small ring, around the star. It's speculation, but there is nothing that denies it. Prove me wrong, anybody.
I do think that the supernode theory is the most likely.
EDIT: Clarified my post a little bit, so that Snail can understand it too