You're assuming the factory did what was best, and that's rarely true. They typically do the cheapest thing they can get away with, which is probably why the rails weren't continued for the entire length of the car to begin with. The roof structure being present in that location, along with a lack of suspension points in that area, meant the cabin area was strong enough to meet the factory standards without them.
Welding the floor along the entire length of the subframe connector with two dissimilar metal thicknesses like the US Cartool design does is also very different than having a spot welded flange like the factory frame rails do. It might look more factory when finished, but from a stress/load perspective it's very different. If you want it to look more factory, awesome, go for it. But don't mix that up with "what's best".
There is a limit to how stiff you can make that section of chassis as well, especially using two frame rails in the locations where they get added. You could weld in a set of railroad tracks if you wanted, but the benefit to the chassis stiffness might not be significantly better than a 2x3" box tube. The weight, on the other hand, would be drastically different.
There is a level of improvement that benefits the performance of the chassis, and after that everything is diminishing returns. Personally my somewhat educated opinion is that regardless of which kind is stronger, the difference in chassis strength between the two basic types of frame connectors on these cars is small enough that the vast majority of people driving these cars won't be able to tell anyway.
Yeah, that's not at all true. Certainly not in theory. There's a reason why bolted connections are used for most engineered structures outside of light duty automotive.
Now, that doesn't mean that the MP bolt in connectors are as strong. Their 2 lag bolts on each end of the connector are definitely not as strong as a welded joint, and definitely not over time. They would need larger landing pads, more hardware and captured nuts in the frame rails to have a chance to be as strong. So for the MP connectors I agree, bolting those in isn't as strong. But that doesn't mean they couldn't be with an improved design.
Making the structure more rigid can actually increase the chances of cracking at the joints/seams. You have to look at how much the metal flexes over the length of the chassis under the loads it will experience. Too much flex can cause cracking from overwork, but too little flex can cause cracking by concentrating more load than the base metal can withstand. Two very different processes, but the result will still just look like a crack to the naked eye. The amount of flex and load has to be matched to the capability of the material used to build the structure.
This is false reasoning. Bolts are used in structural engineering because of their load bearing properties, not because anyone is going to disassemble a bridge or skyscraper. That stuff gets taken apart by explosive demolition, so why isn't it welded? Because of the structural properties needed for the joints, that's why.
Yeah sure, in the automotive world stuff gets bolted together so it can be taken apart and serviced, but that is definitely not the only reason. The entire chassis on one of these unibody cars could be riveted together, and very likely with little to no difference in strength if done right. But they're not, and being able to take it apart has nothing to do with that decision. The factory does things for a lot of reasons besides what's best, what's strongest, etc. and that usually comes down to cost.