Can I call 911 without service?

That depends. Cellphones aren't that reliable. But there was a law passed that emergency numbers have the highest priority in the USA so a phone call to 911 will go through if you are within the range of ANY coverage provider regardless of your phone plan. However this is only if a network has service in your area, and the phone is fairly new. (a phone more than 10 years old probably cant do this)

Even if you buy minutes for your phone and you are completely out of minutes you can still make a 911 call.

Some phones even have an (emergency power supply) so a lot of phones can even last up to ten hours after the phone says it is dead and its screen and everything tuns off. if you enter 911 and your phone has this emergency power supply then it will immediately attempt to make the call. Many people don't know this and not all phones do this.

As long as you have decent cellular signal strength, yes. No cell signal, no calls will work 911 or not.

It would only go through if there's reception where you are. It doesn't really matter if you don't have a plan activated, but you need to have reception.

If you're in the middle of a dessert, it's not going to work.

Yes.