‘Who owns the mineral rights underneath Texas State Highways if the original Grantor reserved the minerals in the deed to the state or county and then sold off the adjoining properties? When he sold the adjoining properties, the deed calls the ROW line of the highway as being the property line.’
The answer is not entirely clear from what you describe. Just because you are reserving rights (mineral or otherwise) in a deed doesn’t necessarily mean that they attach to some neighboring property. They can, or they can belong personally to someone.
So, it’s possible that the rights belong to the current owners of some or all of the neighboring properties, or it’s possible that the original grantor still owns those rights. Only a careful review of the relevant deeds, in light of Texas law, could give you a clear answer. And, to be honest, even then it could be ambiguous: this is why we have judges and juries, to decide when two people disagree on how to interpret a document.
However, if you want a more authoritative answer, you’re going to have to consult with a local attorney.