We received official confirmation that 'all Survivors' does indeed mean all 6 of them in all missions that use the term. You should keep this in mind in your planning.
While it won't change Quest 5, I've also considered house ruling that for multi-part objectives that finish with an 'all Survivors' objective, that the 'all' in this case is just the ones that were alive when you completed the other objectives.
Gotcha. All is "all".
Do you house rule this because you found the some quests tough to beat with all survivors alive? I assume so...