Hmm, weird. Whatever, usually you should only use an event that changes anything on a ship when the ship arrived on the battlefield. Changed the corvettes arrival time to check the events and it worked fine for me. Didn't check if the corvette doesn't arrive after both cruisers destruction though.
Perhaps Gamma wing can't use the guns you gave them in FRED.
In the background editor, there's an area called "Skybox Model". There you should enter the model's name (e.g. starfield.pof). You can extract the starfield.pof and the textures belonging to it from the FSPort or Transcend IIRC. If you don't find it, just tell me. I can attach them here if you want.
Ok, time to go to bed, it's 5 am here
