I did a bit more looking into this “occupied” issue. I’m not sure if I am more or less confused.
The best resource I have found is this Supreme Court case United States v Stitt and United States v. Sims.
https://www.supremecourt.gov/DocketPDF/17/17-765/51424/20180626164915138_17-765tsUnitedStates.pdf
I have not read through it entirely. It is rather lengthy to go into all of it.
The shortish version is that the term “occupied habitation” comes from burglary law. They used to use terms like “dwelling house”. The word “occupied” started being used to differentiate certain places where breaking in would be burglary and places where it would not be burglary.
An “occupied dwelling” would be a place where people actually live. Breaking in there would be burglary. But an abandon dwelling where nobody lives would not. A structure like a barn or shed would not be, unless it was actually occupied by someone at the time.
Something can be an “occupied dwelling” even if nobody is in the building at the time, but an “occupied structure” means that there has to be someone actually in the building. The word “occupied” means two different things.
I think that is where we get castle laws that have “occupied habitation or occupied vehicle”. And even the Texas law that has “occupied habitation, vehicle, or place of business”. That one word “occupied” can mean “generally lived in” when applied to a habitation, but also mean “somebody is actually in there” when applied to a vehicle.
Historically, that seems to be the most common reading.
But it is not entirely clear how it is meant in Texas law. Especially because the next line says, “attempting to remove unlawfully and with force, the actor from the actor's habitation”. The word “occupied” is absent when it is clear the person is in the house.
Is the word “occupied” there to limit the castle law to a home invasion where someone is at home and there are people breaking in, or is it far less meaningful word that is a hold-over from old-timey burglary laws?
The more I look at it, the more it looks like WilliamParcher’s suggestion is correct: occupied in this context just means a place where people are living. If the court takes that interpretation, Guyger has a pretty clear path to avoiding a murder conviction. Which feels somewhat troublesome. It makes the castle law rather broad.