Agreed on all three points.I don't think you are going to find an article that addresses this. I think it is perfectly acceptable.
Welcome to the forum.
If the "box" in question is in another structure,,,,then I think 225.30 does apply here.
What he is saying is the basic rule is each building or structure is served by a single branch circuit or feeder. For example if you want three 20 amp circuits to your garage, you can't run 3 12-2 UFs, you pull a feeder.
But it depends on what your box is. A pull box is OK, a panel box is not.
Yes it could. But it could also mean 2 circuits that remain within the same structure, or are run from and to the same structure but are routed outside the bounds of said structure.The key for me is the word underground which could mean a separate structure.
As an inspector we hope you don't tag without a code number to back it up. So far you gave us too little info to tag this.I ran in to a situation today with two 12/2 underground feeds going in to the same junction box. Each of these are generated from seperate 20 amp breakers. This does not seem correct to me but I cannot find anything in the NEC about it. Looking for help!