It's really not that difficult for the purposes of normal discussion.. No definition for terrorist is perfect but getting semantic or pedantic over it serves no useful purpose other than paralysis by analysis.
When the Declaration of Independence was signed, the colonies became a nation state. Al Queda is not a nation state, nor has it ever deemed itself one. The Taliban is/was. But we went after them due to their harboring OBL.
An innocent person is a non-soldier or non-military personnel, a civilian who has no direct ties to to weapon deployment against enemies. Our soldiers wear clearly discerning uniforms, as outlined in the Geneva convention. Terrorists dress like and hide behind civilians. It can get a little foggy, sure.. like defense contractors who build the things but don't deploy them, or CIA operatives.. but the people in the towers and on the planes were just businessmen, going about a normal day, not planning on harming others. I did say that I consider the Pentagon a valid military target, including the non-soldiers there.
Police could be combatants yes, but their role during 9-11 was to assist people, not engage anyone.. the terrorists were already dead.
Intent.. you're kidding, right? It's no different from a court of law, like the difference between murder and manslaughter. Did they intend to kill those people, or just destroy a building without hurting anyone inside, but the people got hurt "by accident"? I think we all know they wanted to cause as much death and mayhem as possible. Their intent and desire was to kill as many Westerners as possible (not just americans in those towers), and do as much property damage as possible too. Maximum destruction and death.
Mongols would probably fit the definition of terrorist. They did horrible things. Were they a "nation state", or a very large tribe? Did they have an official government, or a chieftain with some cronies? That was many, many centuries ago. The world order is a bit different today, why are discussing ancient irrelevant civilizations?.
BR
Are you referring to Psyops? (Psychological Operations) How often do they target nonmilitary personnel (let's just carpet bomb a school or bank) for assassination?