It doesn't mean that if Shivans are the most obvious it must have been someone else...

And yes you are right. We have no idea what

had in mind. Thus we must work what we have. There has been no indication of any third party influence in the Capella incident. Nor has there ever been any indication that there exists anything more powerful than Shivans. In this universe or in the next one from the right.
If we want to speculate what

*might* have done in FS3, then consider this. If they would have added another omnipotent race/force from this galaxy or some other galaxy, it would ruin the story. Everything thus far has been revolving around the Shivans, and they guard the doors to the answers. Adding something that the Shivans are afraid of would be like saying "Sorry guys, we ran out of ideas. But heres a galaxy killin' master race that we highjacked from the other scifi universe so that we might have another war." And I cant imagine

doing that.
The Shivans are the key here. Destruction of Capella was their main goal, or a side effect. Either way, they did it. I belive this simply because there is no indication whatsoever that it was some third party, or even the GTVA.
I bet the "bigger problem" in FS3 you refer to must have somethnig to do with Shivans and the subspace.
