I am not a bible teacher, but I will give it a go.
First of all, Wikipedia Article on Usury. I notice that you quoted the passage from the King James version. I would say the definition of the word, "usury" has changed over the course of 500 years. I also did not read the 1545 Act referenced in the Wikipedia article. (I have no idea if the King James use of the word "usury" condemns the action like today's use of the word "usury" would.)
(I generally see the condemnation against usury as against rich people taking advantage of poor people.)
The issue in question is the Master gave the Servant a job: Here is $20K. Go an invest it. Did he double his money like the other two servants? No. Was he supposed to invest it? Yes. Was he afraid of his boss? Yes. Clearly, as you said. Now he has to give an account of the money right after the other two did well and doubled their money.
Did the servant have any right whatsoever to pass judgment on his master like that? No. He had a job to do, but he did not do it and now he was trying to justify himself by condemning his boss's actions.
What does Old Testament law say about Usury? It is written in the Wikipedia article. One verse says you may lend to a foreigner with interest.
This might be similar to how you might feel about loaning money to close friends and family members. You might not charge them interest, but you will expect your money back. However, if you were loaning money to a stranger, you are much more likely to draw up a document stating the exact details of the loan, including the interest rate.
What is the context of the prohibition against usury?
I do not know anything about ancient Jewish society. Is a rich person loaning money to a poor person expecting nothing in return simply to be generous? I wonder how the rich person profits? Simply from being generous? If the poor person profits from the rich person's generosity, did the poor person traditionally give something on top of the repayment? I don't know. I'm just asking.
If I lent you $100 to go play blackjack or something, I am really expecting to see substantially less money back, if anything at all. However if you have a great night playing and you have $200, you will certainly return the $100. You might even return something on top of it.
Long post. Does any of this make sense?