Would you rather train a smart horse than a not-so-smart one? [Note -- I didn't say 'dumb horse'!
I don't have a real thought out answer to all these questions. But this one stuck out to me because I have 2 geldings, half brothers (same sire) with completely different mind-sets.
My Curly gelding is super laid back, easy going, not a care in the world, but he's the type of horse that you point, kick and he'll go. He doesn't fight with you, he's a very easy horse to ride and is a sweetheart. He's not "dumb" but...I don't know, it's hard to explain, he just seems to not care as much for what he does when I'm working with him.
His half brother on the other hand, a Curly/Spanish Mustang cross, is a highly intelligent horse. He will test you, push your buttons, and test the limits before he settles into his work. He KNOWS what he's supposed to do. With my mom, he can be naughty. With me, he's usually good as gold because he knows that I won't put up with his crap. He catches on very quickly and the stuff that he learns, really sticks. But his intelligence can play against him.
If I had to pick a horse to work with, I'd much rather have a super willing horse that is easy to work with, over a very intelligent horse that tries to out-think you.
I hope this makes sense. I very rarely ride the purebred Curly, the crossbreed is my main choice between the two because I know that he isn't going to be a butt with me, but I see a lot of frustration come out when other people try to ride him. He is a GREAT horse to work with, as long as your experienced with horses. If not, he'll out-think you.
I'll come back to the other questions with my opinions later.