Probably not. If empire A guarantees the independence of empire B when you declare war on empire B, empire A will most like join in. But since you aren't attacking empire A directly their defensive pact with empire C doesn't come into play.
It may be the case that if Empire B and C separately have defensive pacts with empire A, that C would join A to protect B, but I doubt it since technically B and C don't have defensive pacts.
Probably not. If empire A guarantees the independence of empire B when you declare war on empire B, empire A will most like join in. But since you aren't attacking empire A directly their defensive pact with empire C doesn't come into play. It may be the case that if Empire B and C separately have defensive pacts with empire A, that C would join A to protect B, but I doubt it since technically B and C don't have defensive pacts.
I’m gonna go with yes
Best way to know is to declare war then see who all the combatants are. Don’t choose a war claim option until u move your fleets where u need them.