I can't find any summmon signs in Dark Souls 2, so I was wondering, could that boss could be defeated if I got summoned?
2 Answers
No, bosses defeated while you are summoned as a phantom are not counted towards your own world's progress.
No. The boss is only gone in the world's master eyes. When you defeat a boss as a phantom/shade, you are teleported your world to where you were in your world when summoned, with full estus, HP, and spell uses.
This can be use to be fully prepared for a boss in your world!