The research articles were interesting, but I didn't find them surprising at all.
On the first article, I would have guessed without a doubt that dressage is the most stressful for the horse, without even reading the article, because dressage requires such perfection from the horse and he doesn't have an outlet for the stress. I think dressage even looks stressful for the rider! The other two disciplines- eventing and vaulting allow the horse some freedom of movement to move out and the rider is not micro-managing their every move and asking for complex control of their body while changing gaits and tempos. I did find it kind of sad that the horses were kept stalled and only got one hour a day of exercise, but maybe that is common.
The second article is also common sense because horses are herd animals and of course they are more stressed, worried, and less happy when stabled by themselves. Two together is definitely better, and if they put them in a herd of several horses with room to move around and plenty of pasture, I bet the horses would be happier still.
So I guess the moral of the story is horses don't like isolation and stress. Who does?
I bet trail and pleasure horses who are kept in a herd environment are amongst the least stressed horses. Heck, that probably extends to broodmares and foals too (at least up until weaning time).