"Japan's actions towards Hong Kong and China could be framed as a non-Western rightist approach."
"Rightist" implies pro-Western by definition. Recall:
Rightists believe that non-Western societies should feel fortunate and grateful for all that Western civilization brought them
...
Leftists, on the other hand, believe that non-Western societies justifiably feel wronged and resentful for the ideas that Western civilization forced them to adopt
So I suggest the accurate terminology would be "Japanese cultural supremacist", which would not be a form of rightism so long as "Japanese" refers to Japan in contrast to the West. Only if Japan were presenting itself as
more Western than Britain (and hence more beneficial to Hong Kong than Britain, but hence taking the position that Britain was beneficial also) could it be described as rightist. But the text appears to suggest the former narrative rather than the latter:
Their propaganda also pointed to the pre-eminence of the Japanese way of life, of Japanese spiritual values and the ills of western materialism.
"surely rightist traditionalists within Japan would also have supported dismantling Western colonies and removing Western influences from them?"
As per what was explained above, rightists would not have supported removing Western influences from colonized lands. They would have wanted Japan to take over the colonies, but to then apply the Meiji Restoration approach to those colonies:
https://trueleft.createaforum.com/colonial-era/shimabara-rebellion-the-christian-revolt-that-isolated-medieval-japan/msg13827/#msg13827which would mean
adding Western influences. A good example is what was built in Taiwan under rightist Japanese rule:
https://en.wikipedia.org/wiki/Architecture_of_Taiwan#Period_of_Japanese_rule_(1896%E2%80%931945)









"Since Japan lost, we never truly got to see whether the Greater East Asia Co-Prosperity Sphere would have been a nationalist-socialist anti-Western alliance, or merely Western colonial hegemony replaced with Japanese hegemony."
I would divide it into three possibilities rather than two to make things clearer. If Japan had won, we could have had:
1) Japan promoting each former Western colony's local culture in each respective former Western colony (now independent countries)
OR
2) Japan promoting Yamato culture in all former Western colonies (now Japanese territories)
OR
3) Japan promoting Meiji culture in all former Western colonies (now Japanese territories)
While 1) would seem to correspond to what you call "anti-Western alliance", both 2) and 3) would probably fall under what you call "Japanese hegemony". However, only 3) can be accurately called rightist.
As for whether 1) or 2) is better, this depends on how we qualitatively judge the content of the cultures in question on a case-by-case basis. There is no requirement that we always support the more indigenous culture purely because it is more indigenous. We support whichever culture is qualitatively superior by our standards. This is also why we frequently defend folkish imperialism:
https://trueleft.createaforum.com/true-left-vs-false-left/true-left-breakthrough-folkish-imperialism/