I don't know if NND would really count here though because of the xenophobia you mentioned. She was never advertised for Japan either, she was advertised as "First Korean vocaloid". The Japanese bank, as far as I know was just a additional VB they added to see if it would help her popularity(I wonder if her ENG VB had released, she would have seen decent use in western communities). It was Korea where she had the most popularity.
(I am not sure of how popular Gachapoid is exactly, but I would assume he is popular in some way simply because of how he is based on a mascot. But that is just a guess.)
I was not around to see the late V2-V3 era, but most of VY2's popularity was covers. He barely has any originals as far as I can see. Exit Tunes did commission producers for Mayu songs, but she still had some use in originals(Though she was relegated to covers a lot as well).
This part is really going to boil down to pure opinion, but, the colors are not really good to me. The yellow and brown-ish grey(I am not sure what the color is exactly) are memorable only because it is probably the only time I have seen those colors used. That is because they are generally not used much because it is very difficult to get them to match, and part of me wonders if someone can get them to match at all.
The cube is just okay to me. It has lore, but it is kinda generic. For the longest time I thought it was the NND logo because it was not distinctive.
I have to argue that his art is not distinctive from the Zolas, and that the Zolas are much more distinctive because of their art style. Even if you do not care for Amano's art style(which is a valid feeling), it is a breath of fresh air from the usual Anime style that most Vocaloids are designed with.(Not that anime styles are bad, I love it and it is a good style, but if we are talking about distinction, the Zolas are much more different).
While I agree that their design itself is not memorable, as each one is just wearing a different colored shirt, the intention was to make them as concept art(Which Amano is famous for) and to encourage others to make designs of them. Even if Carnelian's design ended up being the most popular(Something they share with Roro/VY2), the point was to make alt designs of them, and I think the art succeeded in that.
I personally think Kokone is much more distinct because of her color scheme, but that is again, me just having trouble with Yohioloid's washed-out(I think that is the correct word for those kind of colors) color scheme.
I agree. I am surprised to hear about Maika selling poorly considering she was designed to be a multi-lingual VB essentially, but I guess that was not enough. I agree that the source on Fukase seems odd. His voice is very unique, he is bilingual, and Sekai no Owari was incredibly popular at the time. But again, it could be a Maika situation or something like that.
I never knew the west considered Lapis unpopular. She was definitely not popular in the west, but in Japan she had many originals and was very popular from what I saw.
On the topic of Lapis(who is a fairy), on an
official Vocaloid 3 poster, she is apparently fairy-sized and not human-sized. I am not sure if this is canon or just a design choice, but I thought it was very weird and funny, so I wanted to share it
I think I will stick with human-sized Lapis, even if that means it is a head-canon.