It would be a lot simpler to have missions check if the conflicting mission/event was done to then add the new Shields Up effect, because the current way overwrites any vanilla changes that may happen to those missions.
Example:
mission "SU check number 2"
landing
to offer
has "event: temporary cease fire"
on offer
event "shields up temporary cease fire"
fail
event "shields up tempoarary cease fire"
government "Free Worlds"
"attitude toward"
"Deep Sky" 0
government "Deep Sky"
"attitude toward"
"Free Worlds" 0
It would be a lot simpler to have missions check if the conflicting mission/event was done to then add the new Shields Up effect, because the current way overwrites any vanilla changes that may happen to those missions.
Example: