I'm in WA State.. my solution is to carry a mini 14 with loaded mags not inserted.. legal here. I carry it and not a shottie in case I am in a mass shooter situation while about town. 5.56 penetrates soft body armor
What is the source to the link you provided? I selected Home but no URL found?According to this...there are no restrictions for long guns in a vehicle in Texas.
Texas Gun Law FAQ
I always understood that as long as the gun is the legal length (barrel) there were no further restrictions. I used to carry my pistol grip 1300 Defender...loaded and chambered...behind the seat of my truck until I got my CHL; then I started carrying a pistol.
If you're still unsure, just give TXDPS a call. They'll tell you straight up if its legal or not.