And if you ever crash the machine, the rails will move out of position, without the rail guide in place, and Doweled in place, bolting by it's self would not be good enough
That is correct, your design is better, but harder to build, note they are using 2 Ballscrews on the Z Axes, main carriage
Here is a PDF that may help with some parts of your build, it's quite old information, but plenty to be gained from it