You're measuring them by yardsticks that don't fit.
They were not intellectuals, not particularly well educated, not imaginative. They did live their lives day-to-day, in a kind of survival mode, until after the war when a certain 'normalcy' and regularity could be achieved. They knew the lay of the land by then. And it was about them. So what? That's who they were, and they had good reason to feel at bay with the world. It's pretty simple from my view.
I've decided that we here have different motives in discussing historical figures. I have no interest in personally liking an historical personage, certainly do not want to engage in hatred of them (the dead).
I certainly don't hate these two or need to personally like them. I'm interested in seeing how they fit into their time and place. And one thing I find interesting is how little they seemed to pay attention to the lay of the land, so to speak.
Intellect has nothing to do with it, really. They just didn't engage in basic "if I do this, x will likely happen" thinking that most humans through time have done, which lead to them being continually surprised by the consequences of their actions.
Like you, I don't buy into the popular image of Wallis as some evil, scheming thing. I think it's more that she didn't really bother to look very far beyond what was right in front of her face. I don't get the sense that she intended to make anyone's life difficult. At the same time, I don't get the sense that she cared one way or the other about what ripple effects she might set into motion or how others would be impacted. She just seems to me to have been a simple sort of...careless. Which is it's own kind of selfishness.
But honestly, there are any number of instances in which any dolt could have seen the obvious writing on the wall, yet those two seemed utterly surprised.
They lived in a time and place in which divorce was considered shameful across all segments of their society. Wallis herself had experienced first-hand how poorly people thought of divorcees. Any rag and bone man pushing a cart around the East End could have told you that there'd be a lot of fallout to the idea of a royal marrying a twice-divorced woman. Yet those two seemed perplexed by just how upset people were at the idea of him marrying her.
David came of age in the midst of a fundamental shift in royal life for the BRF. George V had recognized like no monarch before the importance of good PR. He didn't want to be associated with their WWI enemy; he didn't want them to be at risk of the uprising and assasination that had brought down his cousin and friend Tsar Nicholas. So he changed their name to Windsor and and placed great value on keeping the middle and lower classes on the side of themonarchy. He created the concept of what we now call "royal work" (frequent public appearances bringing them into contact with Britons of all walks of life as opposed to living a life of leisure behind palace walls and holding patronages in name only). He insisted that the BRF, or at least its public face, be one of loyalty, duty and wholesomeness. He drove home to his children than their future depended on that approach. David may not have liked it, but he was taught very clearly what would and wouldn't fly in that context. If he'd given any real thought to the matter, he'd have known there was absolutely no possibility for a British king to abdicate and then remain on English soil enjoying all the trappings of a prince with none of the responsibilities. Yet that is exactly what he seemed to think would happen when he stepped away from the throne.
Then there's the visit to Germany. I think it is important to remember that Germany and the UK were not officially enemies yet. The visit happened in 1937, when there was still a real possibility that the appeasement argument would win the day in UK. They didn't declare war until 1939. That said, anyone with any basic sense of current events could have seen that it was a very delicate situation. The Duke and Duchess just sort of barreled their way in like the proverbial bull in the china shop. Even if they had ultimately turned out to have been on the same side as the official UK position on Germany, there was no way that bypassing the Foreign office to stage their own, rogue, quasi-royal visit would have ever had a positive reaction from the powers that be. David, of all people, should have been able to anticipate how insulted everyone at Buckingham Palace, Downing Street and Westminster would have been...and that insulting them would come with uncomfortable consequences for himself and his wife. But whether it was about the attention, the chance to thumb his nose at London, or actual political beliefs, he couldn't seem to look beyond the momentary thrill of taking matters into his own hand.
In all of these situations, the pair did not seem to grasp that they were the ones who had pushed themselves further and further into "the outs." They wanted an HRH for her, they wanted to have some kind of life in London's social scene, but their own actions kept jettisoning any chance of rebuilding bridges and getting what they wanted. Theirs is a story of self-sabotage as much as it is one of being shunned and disposed of by the BRF.