In that case. Here's some more.
Actually an explaination of how invalid objectives work is the sort of thing that I should add to the FAQ so I'll answer it in that style so I can later just cut and paste it in What is an invalid objective and how does that differ from a bonus objective?
When making objectives in the objective editor you may have noticed the little Objective Invaild tickbox and wondered what it does. To explain let's take an example mission. Suppose you're FREDding a mission where you're going to destroy a Moloch corvette. When the mission starts however you find the Moloch taking on supplies from a Demon class destroyer. Command order you to ignore the moloch for now and concentrate on the demon. Now look at your objectives. Having the destruction of the moloch as a primary goal while the destruction of the demon is a bonus seems ludicrous. In addition to this the Destroy Demon objective will only appear on the objectives list after
you have destroyed it.
This is where invalid objectives come into their own. The game completely ignores the objective until an event using the validate-goal SEXP is triggered. After this the game treats the goal normally. So basically you've suddenly made a Destroy Demon primary goal appear as if by magic.
Goal validation doesn't end there though. Just as you can validate goals you can also invalidate them. For instance at the moment destroying the demon and the Moloch are both primary goals. If you fail to kill the Moloch you won't get the full victory music even though you did better than the mission briefing expected of you. What you can do in this case is use the invalidate-goal SEXP to make the Destroy Moloch goal invalid. The game will now ignore it.
If you're really sneaky you can even have an invalid Destroy Moloch secondary goal as well as a primary one. Invalidating the primary while making the secondary valid will make it look like the Moloch's destruction has changed from being a primary goal to being a secondary one. It's worth noting that bonus goals can also be invalid too. You could have a Destroy Cain bonus which only becomes valid once the Moloch and Demon are gone
As for the debriefing. Just make a debriefing stage that triggers if the Julius is killed. A red alert mission doesn't mean no briefing. It means no briefing on a successful ending. If you fail the mission you should still get a normal debriefing.
Finally what's this about Sunsath not working on your copy of FRED? Remember you choose it from the bitmap list not the sun list (strangely enough).