I use the beta angle when welding in the horizontal position. It is very useful when welding with a welding process that utilizes a flux shielding system. The beta angle allows the slag to "run" out of the weld puddle instead of building up in front of the arc.

What welding standard are you welding to? It makes a difference when it comes to the positions qualified. If you are welding to AWS D1.1, the answer is different from that for ASME.

The same applies to welder qualification relative to the positions qualified.

The question of preference is often dictated by the means available for preparing the weld joint. Bevels can be prepared with an oxy-fuel or plasma cutting equipment before assembly. Whereas if you are going to tack the members together first, you can use a carbon arc gouging equipment to prepare the J or U grooves very easily once they are tack welded together.

Full weld metal tensile tests are performed when required by the applicable welding standard.

In some cases the root opening is the result of preparing the materials, i.e., the plate was cut twice and it's still too short, or it is done intentionally to allow the welded root bead to contract as it cools without cracking. There are times when I insert an annealed steel wire in the joint to maintain a root opening during fit-up to accommodate the contraction (similar to the root gap used in a socket pipe joint.). The wire is removed once the joint is tack welded. This technique is suggested by Omar Blodgett and works very well.

In other instances, a zero root opening is preferred to hold dimensions and the weld deposit is forced to accommodate any contraction during cooling.

I don't know if this helps, but based on the information provided, it is a starting point for you.

Best regards - Al