I actually am an engineer, and have build my share of race car roll cages. The way you spliced the top of the hoop is solid. The only thing that could have been better would have been to drill some half inch holes in the outside tube on both sides of the weld for rossettes. I don't think that would be an issue though.
I will not be critical of your work, but I will say that the design with the hinge has a strength issue. I know you are saying that the hinge carries no load, but if you apply a load from the rear of the tractor to the ROPS, the bar will want to rotate around the pin you have in there. The problem is, the resistance to that rotation is the hinge, which will have little resistance to failure. Same would go if force applied to the front of the ROPS. By no means am I saying that the work isn't nice or well thought out. If there was room for a second pin a decent distance away from the first, that would be a good solution. If you are OK with the compromise in strength, that is none of my business. There is nothing saying it will for sure fail, but it would for sure fail easier than the factory design, which looks like it has two of the large pins. For what it is worth.