While your theory is credible I wouldn't call it evidence, since them not opening the node themselfs doesn't proofe they couldn't have done it.
The shivans technology is better than that of the GTVA in every single aspect. And if the humans can build a knossos then surely so can the shivans.
Also in one of the command briefings Petrarch comments about the shivans being able to use nodes unkown to the GTVA or too unstable to use for themselfs during the great war. And the gamma draconis/mysterious nebula node was not gone, but too unstable to traverse for allied ships, just like the delta serpentis/sol jumppoint wasn't completely gone, but "only" unpassable. After all the knossos only stabilizes existing nodes, it doesn't create new ones.
But no matter which one is the official canon, we are talking about blue planet here. So it doesn't matter much what volition decided (and neglegted to share with us) to be the real reason behind the shivans actions. What matters is Darius' take on it, which we will most likely learn in BP3, or maybe even in WiH....