Not with this kind of attitude for sure. EU can at least send a strong signal by doing concrete actions. Sanctions against American corporations and individuals, travel restrictions, SWIFT ban. These will make Trump think twice before waging acts of unprovoked aggression.

EU has to focus on its hostile neighbour to the east. I can see you are no fan of the EU but be realistic

Sanctions against someone they need to contain Russia?

Trump and the US has never shown to care about this. The current US gov seems fixated on attacking the EU and trying to break it up. If they want to go to war, EU won't be able to stop them. Perhaps if they gift Trump a plane, though.

No, but it can freeze the assets of its perpetrators.