See, I think Harry has self-esteem issues and henceforth feels the need to describe himself or think of himself as an idiot. He also often uses it to excuse poor and lazy behavior, which isn't great.
I agree he often plays the part of an idiot to fool his enemies, but I don't think it's as Machiavellian as someone constructing a persona. I think its far more organic than that. Harry just doesn't want to think of himself as intelligent and calculating, because that would force him to acknowledge his choices more than he is comfortable with. I also suspect that Harry has grown up in environments that at times made him want to hide his intelligence, so it was easier to pretend to be a fool. This is fairly evident because he goes from listing the benefits of study and being a great student and loving his work as a wizard or a PI to pretending he is just an average joe, just a guy who is like everyone else. And it's pointed out to him throughout the series that he is doing absurd mental gymnastics to connect that logic. Many have reminded him he isn't at all average, and the self-delusion is far more about Dresden's own issues than his mental capacity.
As for the doughnut incident, I think he was betting on Eldest Gruff being friendly, which he rightly assessed after a conversation. If he thought of Eldest Gruff as more hostile, I suspect he might have had a different boon to ask of. He also makes the kind of request to give Eldest Gruff some deniability so that he will not suffer so much of Titania's wrath, which is both polite and and kind. This earns him goodwill with Eldest Gruff, and perhaps by extension those friends of Eldest Gruff. Harry is smart enough to know all this, even if he didn't plan it and thought of it in the moment.
I think it's semantics to say the sub-conscious is smarter than the conscious. It's the same thing. His Id just happens to be more pragmatic. This can mean at times it appears more clear headed, although as Harry points out it just as often means it acts predictably and acting on instinct doesn't trump logic and reasoning.
I agree, Harry could have asked for a treat to summon the Za Lord's guard (although considering Eldest Gruff's skill with magic and combat experience, I wouldn't be so sure that he would be that easy to put down). I don't think I am entirely convinced he or his subconscious had thought of summoning the Guard, but that head canon could easily work.
The reality of course is that the line about getting a doughnut is tension relief; i.e. comedy. Jim creates a series of very tense moments, and releases that tension by creating a very silly moment. Great for an audience.
But I wouldn't be so sure that it was just stupid though. Remember, the Fae have to give and receive gifts of equal value (at least, that's the rule established in Grave Peril and elaborated on later in Ghost Story when Harry and Lea discuss the trade of Harry's memories). Balance is crucial. Had Harry not asked for a gift of equal value, it wouldn't have met the value of his boon and Summer would have been still in his debt and not meeting their word and obligation, and so he needed to ask for something of the value that the boon was given for (i.e. the help that he gave Summer in Proven Guilty).
I do agree that Harry makes stupid mistakes though, plenty of them. Harry would be first to admit that.
Butter's isn't as silly as Harry, he wouldn't even think to do something so strange. Butter's could ask for something valuable but still fairly low-tier though, like providing a distraction when he needs a getaway.