"Learn from me, if not by my precepts, at least by my example, how dangerous ...... than he who aspires to become greater than his nature will...

In answer to your first question, Victor's quote is a desperate request to turn Walton from his ardent quest for knowledge and recognition before he becomes like Victor, someone who has lost everything, including hope.  Victor knows that by telling Walton his story that Walton will see that man can discover formerly unimaginable truths, but Victor wants Walton to see that his precepts--beliefs--and his acting upon those precepts have produced disastrous results.  The last part of his statement almost promotes a provincial philosophy. Victor says,

"How much happier is the man who believes his native town to be the world. . ."

Victor truly believes that if he had been satisfied with living a relaxed, almost "country" life in Switzerland that he would not be in such dire straits.  The last part of Victor's statement shows what Victor's true motivation was in creating the monster.  He was not seeking to make some great contribution to human science for the benefit of others; rather, he--like Walton--wanted to make a discovery that would bring him admiration and fame.  This arrogance on his part is unnatural, and so nature has to put Victor in his place.

Secondly, Victor wants to tell Robert the whole story of his creation, but he does not want to do so briefly.  By relating all the events leading up to and following Victor's "successful" creation, he hopes to prove to Walton that he was once just as Walton is when Victor meets him.  If he tells Walton too early that he was able to create life, he is afraid that Walton will "run with" that, and not heed the warning that Victor is trying to impart.

Finally, the middle of the quote is an example of inversion or emphatic structure.  Shelley reverses our normal subject-verb order by writing "how dangerous is the acquirement of knowledge." She starts with an adverb and adjective, then adds the verb, and lastly the subject.  She does so to emphasize the horrific nature ("danger") of seeking too much knowledge.  Notice that she does not begin with the subject (acquirement) as Victor mistakenly did.  He focuses first on gaining knowledge rather than the dangerous consequences of his quest.

The last part of the statement is also a good example of antithesis (parallel structure of opposing ideas).  Shelley contrasts two different types of thinking by using the same word order and structure.