After the massive frustration-fest I went through to barely squeak by this one, I think the main problem was definitely how far away everything is from each other. The one time that I managed to haul myself to the Ravanas faster than any other attempt, I managed to disarm them with the Keeper at 50% or so...but by the time I made the incredibly lengthy journey back in the other direction toward the Liliths, the Keeper was barely hanging on. At that point, even the bombers and Moloch posed a formidable threat, and I was insanely lucky that the Keeper jumped out with 2% hull remaining. And like you said, from the time I worked on the Ravanas to the point when I was able to disarm the Liliths, I had a full Shivan wing right on my tail that I was never able to outrun. Moving all of the Shivan ships closer to the Keeper (or better yet, closer to each other) might go a long way toward fixing things, as would getting your fellow Vishnan ships to lend a hand with one or two of the Liliths. Another fix might be to simply move the player start point far closer to the Ravanas at the beginning, so they could be disarmed before doing nearly as much damage to the Keeper.
Also, like someone mentioned in the main release thread when I was asking for advice, this mission would be far easier as-is if you were still flying a Terran ship. A bank or two of Trebuchets would do wonders for keeping you near the Keeper while still allowing you to de-fang the Shivan heavy hitters. Perhaps another alternative to changing the mission would be to introduce a Trebuchet-style missile for the Vishnans, although I don't know the logistics of doing so at this point.