I'm no expert but I-35 would probably be improved greatly by just adding another lane.
Adding lanes to a freeway does nothing to speed traffic. Shutting off exits would speed it up more.Limited access highways are needed and toll roads fill that need. Houston has greatly improved it's traffic flow in the past 15 years by the tollroads on the beltway.
I-35 needed another lane 20 years ago. Maybe it will be finished ten years from now. But by then, it will need another lane or two. After the current expansion, there's no room on the existing right of way to add anymore lanes. By 2040 there will be over forty million people living in Texas (double the current populuation). The existing Interstate highway system was designed for a life of 30 years. It has now been almost 50 years since the Interstate Hiway Act was signed. We need to plan for the next 50 years not just incrementally add to the existing infrastructure.
That depends on how entrance/exit ramps are designed. The entrance/exit ramps on I-35 between Austin and Hillsborough are atrocious and very dangerous with very short and abrupt merges from the access roads to the freeway. I-37 has much better ramps
Most of the recent accidents on I-35 have happened where there are only two lanes and you have somebody trying to pass a semi. Having at least one more lane on the present I-35 would do a world of good because it would allow for a very usable passing lane on the left.