With a little bit of scripting (like the Sath opeing up before even clearing the jumppoint) the whole battle was over in 20 seconds....
If you'd like I can attach the mission file, so you can see for yourself.
See, this emphasizes the fallacy in your set up. You're assuming that everything will go perfectly for the Sath, and perfectly wrong for any opponents. Combat does not work that way.
Not at all.
Because in my little FRED example the Sath jumped into the waiting guns of the GTVA ships. Hardly a perfect setting for the Sath, right?
But despite the Sathanas jumping into waiting guns it still took only between 8 and 11 % damage (I repeated the experiment three times... each time the same outcome, except the 3 % variation in the damage the Sath took).
The only thing that might be a problem for the Shivans is, jumping in at exactly the right distance. And exactly the right distance is still a very big window, considering the massive range of the BFRed.
The jump-in point is the only point of failure. If the Sathanas jumps in within range and angle to hit the GTVA ships with her main beams, it's game over. There is no room for best or worst case. It's simply over.
The AWACS might change that outcome, but since the 14th battlegroup didn't have one with them, that's a moot point.
The scripting I did wasn't to make the battle more favourbly for the Sathanas, but simply to overcome the stupidness of the AI.
Why should the Sathanas blast the debris of an already destroyed ship, when there are other ships in range, that can still fight?