Is God the source of evil, according to these passages?

In these verses, the word "evil" is ra. This word does indicate moral evil elsewhere. But there are meanings for this word like "adversity" and words of similar nature. Ra can therefore be used in both senses.

Now with this in mind, how do we determine the proper translation of ra in this case?

The answer is simple, once we consider the literary parallel in the verse in question. Note the antithesis in the first part of the verse from Isaiah: light/darkness. The second part of the verse must also be therefore reckoned as an antithesis. The word we translate "prosperity" is a familiar one: shalom. We commonly translate this word "peace" - but it is NEVER used to indicate moral goodness, the antithesis of moral evil. We must therefore translate "ra" in terms of its specified antithesis, and that is why it is thoroughly proper to give it the meaning of calamity/disaster/adversity here.

Some may argue a philosophical objection that God should not have the right to cause us adversity, but the text is speaking of judgment and discipline, not moral wickedness.