What's your interpretation of the origin of the Celestial Breed? Are they Chinese? Japanese? Are they even related to Imperials at all, or was that a lie to hide the source of their celestial egg?
Oh interesting questions! I will have to circle back to this once I actually go through all my (many) notes, but I was actually keeping track of the Celestial/Imperial interbreeding specifically.
Just had a quick look at my notes, at those on the Japanese Celestials in particular, and my thoughts on the origin of the Celestials and their history can best be described by repeatedly hitting the question mark:
So I am really not sure about the origin, though I am tempted to say China; purely for the reason that irl (to the best of my knowledge) dragons spread from China to Japan. There's also this:
“the story of the Yellow Emperor of China, the first Celestial dragon, on whose advice the Han dynasty [202 BC – 9 AD, 25–220 AD] had been founded” (b.1, ch.8, p.204)
but it is a story that may be rooted in truth or just straight up invented.
Now I could also headcanon my way into saying that they are from Bhutan since the official name Druk yul can apparently mean "the Land of the Thunder Dragon", and maybe then stolen from there or that the Celestials were bred from more feral dragons with a lesser version of Divine Wind found in the Bhutan area.
As for their relation to the Imperials: I do think that the Celestials and Imperials are quite closely related and will quote Howe for that:
“Brief observations by travellers have yielded only a handful of incomplete details; we know that the Imperial and Celestial are distinguished by the number of talons on their claws, which are 5, unlike virtually every other draconic breed, being four-fingered; similarly, their wings have 6 spines rather than the 5 common to Asian breeds. In the Orient, these breeds are popularly supposed to be highly superior in their intelligence, retaining into adulthood that remarkable facility of memory and linguistic ability which dragons ordinarily lose early in life. [...] That the Celestial is closely related to the Imperial may be inferred from the few illustrations we in the West have managed to obtain of this breed, but very little else is known of them.” (1.1)
Now of course he could be wrong, since this was written about 10 years before he met Temeraire and therefore an actual Celestial and everything he writes is based on accounts of other people. I'd agree though that they are too similar for them not the be related. I would also very much agree that the whole 'two Imperials can sometimes produce a Celestial egg' is a lie used as a cover up.
Ideally this is where I'd say hope this answers all of your questions but I am confused/torn about this myself ^^' Thank you for asking though! This was fun to think about :3