This is Masashi. Sasaki(my clivia friend) & I went to Mr.Nakamura's plantation the day before yesterday, and asked Mr.Nakamura about your questions. He gave me some information for you.
He agree with your idea that all the meristematic growth is affected at the same time. He also thinks that there is a trigger in spring that causes the variegation to appear. The appearance of variegation is related to temperature or day length, he just thinks so from his breeding experience, and wants to confirm this fact by continuous watching. About original Akebono parent, it is said that Shimizu plantation (I have no idea about the name) introduced the plant to Japanese market before, but it is not certain. I just think that Japan has a history that people made efforts to breed interesting variegated clivias(C.miniata), may be someone found Akebono by chance and the offsprings were welcomed to enthusiasts. Mr.Nakamura said Akebono will be inherited with almost 100 % possibility by crossing a good yellow.
The below is Sasaki's message about Akebono.
Hi, Abel! This is Shigetaka Sasaki. Nice to meet you. I want to share my ideas about Akebono with you and everyone.
I want to breed like following method if I have Akebono yellow. I have no data re:Akebono, so my ideas are baesd on Nakamura's explanation and the data of striped clivias. You said your Akebono Yellow is F2 ( may be (Akenobo x Vico Yellow) x Vico yellow ), I think the plant is homogenious (aa). Therefore, you can get 100 % Akebono Yellow if you use the pollen of yellow flower which has homogenious gene. I heard that Sometimes F1 of yellow flower will produce 100 % orange, so please use homogeniuos yellow pollen. I recommend the pollen of Vico Yellow or hybrids of Vico Yellow. I think you can get 100 % of Akebono yellow by self pollinating, but one thing is important that clivia has a tendency to produce less seeds and weak plants when it is selfed. I don't mean that using other flower's pollen is better, self pollinating also has the merit that hidden genes will come up and give us a new possiblity of good flower and foliage since the genes become homogenious. If you just want to produce a large number of Akebono Yellow, I recommend the method of " Akebono Yellow x Vico Yellow". When you want to produce new plants by using the pollen of Akebono stripe, I presume that F1 is 100 % non variagation, F2 or F3 is 20% to 30% Akebono stripe, this idea is based on the data of usual striped clivias. If I have a Akebono Yellow, I want to produce many various types of F1 by using the pollen of gardenii, caulescens, nobilis, good miniata, daruma( wide leafed miniata ) and so on. I hope you can get good results of Akebono Yellow breeding.