The problem is not with ship-guardian or ship-invulnerable, it's with the less-than sexp. Or, alternatively, it's not a problem at all; it's a design decision. If you have ship-invulnerable set to go off when a fighter's hull strength drops below 50, and said fighter gets hit with a missile that drops him from 75 to 0 instantly, it's too late. The solution is to ship-guardian him when the mission first starts, and then use a ship-invulnerable sexp later on if that's your thing.
That's why I asked for confirmation that the problem was actually with ship-invulnerable. I'm pretty sure that most of the reports of it not working are due to this exact cause rather than any actual problem with the SEXPs.
A look at
Love the Treason..... pretty much confirms that. Simply adding a ship-guardian at mission start would prevent Snipes from ever being killed in that mission.
Incidentally as far as I can see the mission Jim Boone was completely unaware of the existance of the add operator option in FRED. Every single event I looked at seemed to work like this
Event 1
when
-trigger
-Do one Thing
Event 2
When
-Same Trigger as Event 1
-Do something else
I can't think of any sensible reason for making 26 seperate events for the destruction and sabotage of the NTF wings when it could have been done in 3 or 4.