You missed the point. You refer to previous valid code specifications that have since been changed. Previous work done legally under existing code is not the issue here. Work done illegally must meet current code.
I'll use your own example. You state that prior to 1996, NEC code specified 3 wires, after 1996 4 wires. If wiring were done in 1997 and 3 wire was used, that is something that most states would require to be fixed.
I'm no expert on Texas, have never even been there, so I suggest you call your local building department and ask.
By the way, on the sold "as is" issue, if you know of a problem such as faulty wiring, you must disclose even if you sell the house as is. That clause wont always protect you in court.