If it targets others, it is. Some countries have ideas like that only non-state entities can commit terrorism, but that is nonsense. My own country's laws don't distinguish between state and non-state acts in its terrorism legislation, for example.
Yes, distributing explosives into the public is terrorism, regardless of if they're discovered or not. I'll add that this mossad agent is also, by definition, engaging in terrorism since he's trying to cause mass fear that they've distributed explosives worldwide.
If it targets others, it is. Some countries have ideas like that only non-state entities can commit terrorism, but that is nonsense. My own country's laws don't distinguish between state and non-state acts in its terrorism legislation, for example.