I’m not sure, but I think they also released the regular version in Australia. Whether that was the US or UK insert I dunno. Maybe I’m mistaken though, but considering the Australian variant with Game Boy Color insert is pretty rare for the Ancient Mew and Legendary Birds (it took me more than three years before I was able to find the sealed Moltres with Game Boy Color insert for my Moltres collection for example), I would assume another more commonly to find variant was released in Australia as well.
Yes. The Korean version was released recently in September 2020. I actually just opened two sealed version to send to PSA, hoping for some 10s to complete my sextet of PSA-10 Ancient Mew versions. 
Here pictures of that sealed Korean version.
Yes, those six Ancient Mew versions currently exist. Four of them are discussed in this excellent, but outdated, Efour article. And the Japanese ©2019 and Korean ©2020 versions were released when the CGI remake of the first movie was launched in cinemas in Japan and South Korea in July 2019 and September 2020 respectively.
Greetz,
Quuador