Can I literally understand the first clause as "if a wise man is a man, then he will be useful"?
Not exactly. In "only be useful as a man", the meaning of "man" is very precise and is key to understanding what's being said. In that specific use, "man" means something like "morally courageous person". So in fact what is being said that if the wise man is useful (and that allows for the possibility that you may have a wise man who is not in fact useful), his usefulness will be precisely because he is a man, i.e. a morally courageous person.
What does "office" and "dust" mean?
Here, office means "job" or "role", and is referring to the job or role of "stopping a hole..." which in this context is seen as something quite low and not very demanding in terms of moral courage. And then dust means his lifeless remains after he -- the moral being, the man -- has died and departed.
So what is being said is something like this.
A wise man is truly useful not because he is wise, but rather precisely because he is "a man", i.e. morally courageous. And an essential part of that moral courage lies in not allowing himself to be moulded (i.e be clay) according to other people's beliefs. Furthermore, he will not avoid difficult decisions by, for example, simply ignoring problems or applying a quick but ineffective fix ("stop a hole to keep the wind away"). A moral man would not stoop to do a job that involved such low and dishonorable behavior. Instead, he would rather leave that kind of job -- that "office" -- to his own corpse, when he had become mere dust.
Does the second clause make the "not...but..." construction?
I'm not sure about this one. Does the above answer it?
tl;dr The first sentence is much easier to understand, at least without context.
There are several different types of buses with their own dedicated lanes like a network in the city.
What "their own" does is emphasize that each bus has its own group of lanes, not shared with any of the other buses.
If "their own" were not there, it could become unclear if the lanes were dedicated to the buses as a group (the entire group of 5 buses gets 30 lanes), or to each individual bus (each bus gets 30 lanes, or 150 lanes in total).
There are several different types of buses with themselves dedicated lanes like a network in the city.
It's hard to imagine a case where "themselves" would work here, since "themselves" is a reflexive pronoun.
But here's a very contrived context where it could work:
Public transportation on the island is broken down into two types of transportation: buses and boats, each with their own dedicated set of lanes [the buses have a separate set of lanes from the boats].
There are several different types of buses with themselves dedicated lanes [each type of bus has its own set of lanes] like a network in the city.
What's happening here is that we're emphasizing layers of ideas. The buses are separate from the boats; and within the buses, bus A is separate from bus B.
I'd still much prefer the first one, though. I'd only use the second to avoid the repetition of "their own."
You mentioned in a comment an example in medicine:
Patients should be able to refer themselves to a specialist in such circumstances.
Here, "themselves" is being used as a reflexive pronoun together with a reflexive verb. The statement is that the patient is referring someone to a specialist, and that the "someone" is the patient.
It's not the same as in the bus example, where it only serves as emphasis. Here, it would be grammatically wrong to use "them"; instead, use "themselves," since the subject is the same as the object.
When in doubt, imagine replacing the subject and object with people:
Adam referred himself to a specialist.
Adam referred Adam to a specialist.
The first one sounds much less weird than the second, right?
Best Answer
(Native American English speaker here.)
They're all clear, they're all normal English, and it's nearly impossible to say which one is best.
These are also equally good:
If you want some factors to consider in making a choice, which might also shed light on other word choices in other situations, here are some:
These days, it might not be a safe bet that your audience knows what per se means. For an academic audience, though, per se is fine. Note that since per se is Latin in the middle of an English sentence, it should be italicized in writing.
The meaning of “as such” is correct, but in this context it's less immediately clear than the other (all-English) versions. Other people may differ, of course. But I go through a moment wondering “as such what?”
An explicitly reflexive wording—“their own”, “themselves”—adds weight to the meaning you intend here, and probably makes the sentence clearer and easier to read.