At one point I started to discect the Riddle, but I kinda st\]opped, but I'll post what I had so far...

Paragon of a mute usurper, I.
To Occident and Orient adjunct.
Mine avocation, dearth to pacify,
By rubicund umbrage nigh-on defunct.

If you uses “paragon” as a noun then it would mean a perfect model, therefore suggesting that it is a statue of a usurper. (dearth = so rare that it valuable, or that it lacking makes it of some sort of value). This Statue, or what or who it symbolizes united a separated land. The last two lines I get the impression of a job which through a war causes peace. The war was not the first choice…Overall to me it seems that it points to a king who united a divided land, and caused an uncharacteristic war that made peace…

O, keen watchman to mine own, falter not,
To vouchsafe designs most astronomic.
Misplaced pursuit doth leave much truth forgot,
Affinity only one may mimic.

This seems like a narrative of someone who is passed away. This person is talking to a watchman of sorts. It could be a person, but the perfect watchman would be a thing that is relatively immutable, like a statue…(Why am I getting analogies from Alexander the Great and Greek Myth from the first two paragraphs?). So this “watchman” is the guardian of a secret designs which are great. These secrets are… The narrative person, he is asking this statue not to fail “mine own”, who may refer to the next stanza dealing with his “heirs apparent” are they one in the same…It seems as the third line is a warning of some sort, as if there is something fallacious about pursuing the “design most astronomic”, and that fallaciousness is only mimicked (we could also replace mimic and use parallel…) by one person…

I forgot to do revisions based on what you put up there...*forgets most of what he did*