"Not all of the missions succeeded" and "All of the missions didn't succeed" don't quite mean the same thing.
"All of the missions didn't succeed" is really the polar opposite of "All of the missions succeeded."
"All of the missions didn't succeed" = "None of the missions succeeded."
"None" is the polar
opposite of "All"
"Not all of the missions succeed" leaves open the possibility that some, but not all, of the missions succeeded.
"Not all" is the logical
opposite of "All"
When we're negating, we want to use logical opposites. Memorize the logical opposites of quantity terms:
The logical opposite of "All" is "Not all"
The logical opposite of "None" is "Some"
Negating conditionals can be tricky! You are correct that the way you diagrammed the "unless" statement is simply the contrapositive of the way it was diagrammed in the solution and so they are logical equivalents. You are also correct that the negation you ended up with is not a contrapositive of the negation that was in the solution. But your negation is still correct. This comes back to what we're actually doing when we negate a conditional statement. When we negate a conditional statement, we aren't technically creating another conditional statement. What we're really saying is that the sufficient condition can be true even if
the necessary condition is not true. That's not the same as saying that if the sufficient condition is true than the necessary condition is not true. "Even if" does not create a conditional relationship and so the negation is not really an absolute conditional statement. So your negation doesn't have to be a direct contrapositive of ours to be correct.
Here's a link to a blog post that discusses this in a bit more depth: https://blog.powerscore.com/lsat/bid-29 ... -the-lsat/
And here's more of a discussion on "even if": https://blog.powerscore.com/lsat/how-to ... nt-matter/
Hope this helps!