A terrorist is someone who employs terror as a political weapon, a murderer is someone who commits murder. They're not synonymous, although a terrorist may employ murder to create terror, just like they could employ facial hair in a pogonophobics group session.

If the U.S. has an official definition of terrorism, they aren't sharing it with the rest of us.

Regardless, the point of the "invasion or rebellion" exception is obviously to handle circumstances so extreme that there won't be a government to recognize habeus corpus if due process is followed. The U.S. faces no such threat from lawfully dealing with terror suspects.

Yes. The government argues that terrorism is a unique threat that requires extreme measures, such as removing rights from citizens. But the counter-argument is that it IS a unique threat: one that can never be conclusively dealt with. There will never be a time when we can say "we have won; we are safe from terrorists." Therefore, any rights we surrender in this "war" will be gone forever.

In other words, the ubiquitous, eternal possibility of terrorism is the strongest argument for why we should NOT allow "extreme measures" that involve citizens losing their rights, because that loss will be permanent.