Okay so I'm going to take this point by point.
1. I'm not disregarding characters like Dee, or any others, it's my (and many others) personal opinion that Isaac was next. I feel like enough circumstantial evidence points to this being the case, as I detailed in my theory.
2. If we look at popular characters during the ballot era and those that made it into Smash, we can see that Ridley and K. Rool were really the only ballot choices. Inklings were obvious, Simon's a great third-party rep, and Incineroar didn't even exist until 2016. It's common knowledge that Ridley was a huge request for a long time, and when he was finally revealed, Japanese players started calling him "Captain America" jokingly, since he got almost zero support from over there. K. Rool, however, is considered a widespread request, and I believe that he might have been the overall winner, since he places highly on many polls. Additionally, echoes like Chrom, Dark Samus, and Richter are also fan choices, as if you look at the exit poll for the Smash ballot, both Chrom and Dark Samus placed quite highly.
3. Golden Sun sold exceptionally well in Europe. The first game sold 380,000 copies (according to VGChartz) alone in Europe, and the total sales reached 1.8 million lifetime. The Lost Age, the second game in the Golden Sun franchise, sold 270,000 copies in Europe, reaching 1.22 million lifetime sales. Dark Dawn sold much less, only selling 100,000 in Europe, but that game only reached 810,000 copies lifetime (which is still incredible for when it released in the DS's lifespan). So, both Golden Sun and The Lost Age happened to outsell a particular game that features a particular character you might know: Kirby's Return to Dreamland sold a whopping 210,000 copies in Europe, and its lifetime sales are 1.66 million, less than the first Golden Sun. You are simply wrong in your assertions that Golden Sun didn't sell.
4. Why would Bandana Dee be popular in Japan? I don't know, I'm not from Europe, but I imagine it's very similar to how I got drawn to him: his unique abilities, neat design, and excellent home series. It's a taste thing, man.
5. I know that polls don't represent the entire voting population, but it's about sample size. In the exit poll with over 45,000 votes, Isaac placed second worldwide. The only character ahead of him? King K. Rool. It's by far the largest exit poll we have, and is fairly reflective of what we got in Ultimate. The top 10 featured 5 characters that didn't make it: Paper Mario, Bandana Dee, Shovel Knight, Banjo-Kazooie, and Isaac. Of the top 10 that did make it, Bayonetta, Inkling, Snake, Wolf, and King K. Rool, the four returned in Ultimate. This poll is highly reflective of what actually happened.
6. Yep, I've already admitted that Bandana Dee beats Isaac in Japan. He's definitely the most requested over there, so if there was a Japan pick, it'd probably be him. However, he doesn't have the worldwide popularity Isaac has. The Source Gaming poll is very reflective of Japan's picks, but this game's fan service wasn't just about Japan, and Ridley is the proof.
7. I don't think these polls have bias, and I believe them to be reflective of the general community during the ballot era. Just like how political polls don't actually show what's going to happen, they usually can predict a general theme and results, just like these polls.