I am not current with Quantum Destinies; I read parts 1-4 quite some time
ago, but nothing since.  Thus I may be missing info explained in the
stories.
However, I was sufficiently intrigued with this short essay to offer
commentary.

legitimately traced to the first Emperor of the current ruling house.
Over time, the inbreeding that can be caused by this is offset by the
frequency of successful usurpation.  Historically, the Imperial
Bloodline has either died out or been overthrown roughly every four
hundred years.

This is _very_ deviant from the Japan we know, which proudly asserts
that the Imperial line is continuous from mythical times.  It sounds
much more like China.

The current Imperial line has been in power for almost four hundred
years, and is both the most successful and the most vigorous 
seen.

I assume that Japanese history prior to ~1600 CE is unchanged?  Then
the Imperial Line hadn't actually ruled for over 600 years prior to
the current Imperial line taking power (ever since the Fujiwara seized
real power during the Heian period).  So I see little point to the
comparison, true though it obviously is.

This line is derived from the Great Unifier, Oda Nobunaga, who, 
after evading
assasins in his youth, conquered all of his opponents in the 
Unification
War.  Impressed with him, the then Emperor strengthened his line by
marrying his heirs to the sons and daughters of Nobunaga. 
Thus was born
the current bloodline.

It would be very like Japan for the Shogun's family to intermarry
with the Emperor's.  It would be very unlike Japan for the result
to be officially labeled as Nobunaga's line.  Fujiwara daughters
married to Emperors gave birth to Emperors, not Fujiwara.

Only a generation later, the Nobunaga Family
essentially became the Imperial Line, and the expansion of a unified
Japan really began.  Within three generations, the power of the
Shogunates was reabsorbed into the Emperor's hands.

Is it ever explained why the Nobunaga family's policies differed
so much from the (real) Tokugawa family's policies?  Presumably
the same forces which led the Tokugawa into isolationism were
still operating.

Again, the sentences above don't read like the way any Japanese
person would explain the situation.  Why isn't it "intermarriage
with the Nobunaga Family revitalized the Imperial Line, and the
expansion... began" without a second sentence?

2. High Nobles - This is the class that contains the 
descendants of the
pre-Imperial "Shogunate" nobility.

Does this mean all daimyos from the warring states period?
Or only the descendents of the pre-Tokugawa shogunate families?

When the Empire was still expanding, many new
clans were formed when the younger children of these houses 
were sent to
become the governors of newly conquered lands.

This raises an interesting question.  Spanish colonies recognized
a distinct status difference between people born in Spain, and
people of pure Spanish descent born overseas.  What happens to
the status of Japanese born overseas in the QD world?

 The second branch was added three 
hundred years ago
in response to the pressure from the European Merchant 
Companies.

Japan was expanding from 1600 CE to 1700 CE, but didn't have its
own zaibatsu yet by 1700?  Or did, but were ignored, while the
European equivalents (Dutch East Indies Company, etc) weren't?
The former appears to imply that Japan followed a Spanish colonial
model:  direct control, via a viceroy, for the purpose of religious
conversion and extraction of precious metals for shipment home.
Note that the latter policy led to the collapse of Spanish power
in Europe.  Why didn't Japan collapse, given such rampant inflation
at home?

 The last group of nobility was
created a hundred and fifty years ago in response to the technological
superiority that the European monarchies held over the people of
Nippon.

Circa 1850, i.e. just after the inventions of the railroad and telegraph,
Japan noticed a "technological superiority"?  How?  Why?  If they'd
been expanding for 250 years, surely they'd have kept somewhat more
current than the real Japan did?

Remember that China thought European inventions were amusing diversions
and nothing more up through about 1800.  Up until about then, European
inventions _were_ nothing more than amusing diversions.

The real Japan made the best guns in the world in 1600.  If, instead
of dropping into peaceful isolationism, Japan had embarked on warlike
expansionism, wouldn't they have still made the best guns in the world
in 1850?

4. Citizen - The vast majority of the populace of the home islands are
Citizens.  There are also Citizens scattered throughout the 
rest of the
Imperium.

This almost implies a Roman model?

5. Civilian - A civilian is a member of the class that are fully
Nipponese, yet have lost their Citizenship in one way or 
another.  

I'm not sure I grasp how this category developed.  In real Japan,
only Ainu and Burakumin would appear to be candidates for this
category.  The ideology of real Japan is that all other Japanese
people are undifferentiable, but different from the rest of the
world.  Presumably, a divergent ideology of "responsibility to
the Empire" has to exist, and the enforcement of this ideology
has to take precedence over ideological attempts to assure the
Japanese people that they're all the same.  In the real world,
even in the run-up to WWII, homogenization took precedence.

6. Freeman - Sometimes called the "Noble Peasants."  The euphemistic
phrase used for those that are either half-Nipponese or their 
country is
considered to be closely tied to Nippon.  The Koreans fall into this
category.

Koreans are looked upon favorably by Japanese?  What a change!

7. Peasant - Any of the conquered peoples of the Empire.  
They are given
full educations and are encouraged to strive for Citizenship.

Since I haven't read QD recently/thoroughly, I'm not sure who
the other conquered people are, other than west coast native
Americans.  (Presumably, if the Japanese ever conquered European-
colonized areas, the Europeans would receive "honorary Japanese"
status.)  The implication is that the Japanese weren't impressed
by Aztec or Inca or Javanese achievements.

8. Barbarian - Anyone from any part of the world not under the
enlightened rule of the Emperor.  Depending upon which empire 
they come
from, they are granted either Civilian or Freeman status while they
journey within the empire.

Why isn't this "Freeman or Peasant" status?  Are the peasants of
other empires denied visas?