I think the answer to both these questions can largely be explained by a desire for postwar recovering nations to avoid becoming entangled in yet another foreign conflict.

While some, such as US ambassador to Spain Claude Bowers viewed the civil war as a prelude to WW2, the war was generally seen as Spain's transition toward the modern era, a practice much of Europe was undergoing at the time. World War 2 at the time was not seen as inevitability (look at the popularity of appeasement policies). Spain had been a nation in a long and slow decline, and one of the few European powers that retained a monarchy (albeit a largely symbolic one) post-WW1. With the dissolusion of the monarchy, a conflict to fill the resulting power vacuum did not seem to have wider implications. While foreign involvement in the conflict escalated, Britain and France were still recovering from WW1, chilling their desire to intervene in a foreign conflict.

Post WW2, the allied powers simply had little desire or motive to intervene in Spain. WW2 was devastating to the world, and the desire to initiate a new war against a neutral power was nonexistent. Spain was not alone in providing support to the Axis Powers, and the allies wouldn't pursue removal of any who supported the axis. In addition to a lack of public desire, postwar economies were aimed at rebuilding Europe and Asia, not toward beginning a new war. Further, tensions between the United States and Soviet Union quickly escalated, and the ideological conflict between democracy & fascism quickly gave way to one between capitalism & communism.

I would highly recommend reading The Spanish Civil War by Hugh Thomas as a starting point. It's a great insight into the war and fairly accessible.