I will answer what I can, but for a few of the questions I am sure someone else will be able to answer better than me.

Why was the conquest so short

Surprisingly enough, it wasn't. Italy attacked Greece on their own (no German support) on October 28th 1940, and their initial offensive went horribly. They gained some ground initially, but in a few weeks time the Greek forces had pushed the Italians back to the pre-war borders, and the Greeks even mounted a few counter-offensives into Italian Albania, which took place through January.

Eventually the war fell into a stalemate, which was attributed to the lack of Italian equipment for a successful campaign through the mountainous Greek terrain. The Italians tried for a second offensive in the Spring of 1941, but that failed pretty horrifically too.

Later in the Spring the Italians got much needed German support in the offensive, and the Germans pushed through Yugoslavia which took less than 2 weeks, and then then in late April they assisted in the invasion of Greece. Greece then fell to the Germans on April 30th 1941, and eventually Crete fell to German paratroopers in early June.

So overall the Balkan campaign took almost half a year because of the Italians being ill equipped, and because of stiff Greek resistance.

On 6/4/1941, Yugoslavia was attacked from all sides by Germany, Yugoslav neighbours who were German allies apart from Greece and internally by Croatian fascists, the Ustashe. Country's army was primitive compared to German. Initial bombing killed as many as 20,000 in Belgrade. Upon strolling through the country in several weeks, Germans installed proxy government in Serbia. Army disintegrated, king escaped fo Great Britain. Effectively, Yugoslavia was divided between its neighbours and Germany. Germany held Serbia, Banat and Slovenia. Italy, Hungary, Bulgaria took the regions closest to them. Croatia proclaimed its independence and incorporated Bosnia. Remnants of the army retreated to mountainous areas of south Serbia and Bosnia and kept up the resistance of low intensity for most of the war. They were mostly Serbian Royalists and were known as Chetniks. Chetniks low key resistance was due to German rule of killing 100 civilians for 1 dead German soldier done almost exclusively against Serbian population. Germans executed many civilians in Serbia to set an example to anyone that would dare resist. Upon German attack on Soviet Union, Yugoslav communists -partizans - began armed resistance against Germans, other occupying forces and domestic Nazi proxy governments. Their numbers were initially small, but were greatly bolstered by Serbs from Croatia and Bosnia who due to Croatian ustashe (nazis) attrocities (on par or worse than Rwanda + death camps) against Serbian population in newly formed Croatian Nazi State (todays Croatia and Bosnia). Partizans in Serbia left for Bosnia by 1942 as population did not like getting killed by hundreds for assasination type Attacks on Germans. In Serbia Partizans came back in 1944 on the wings of Allied victories in East and west. In mountainous parts of Bosnia and Croatia, partisans held sways of free territory, as majority of them, local Serbs protected populace. Germans and Italians assisted Croatian nazi authorities by sending expeditionary forces to quell the uprising. These failed, even though partisans were decimated. Partizans destroyed fascists without qualms about retribution against civilians as Croatian Nazis were already exterminating Serbian population. They has nothing to lose. In 1943 GB switched support to partisans from royalist chetniks dud to their usefulness. They tied up as many German divisions as the whole of western europe did. Yugoslavia lost 1.7 million people. I forgot to mention that wwii was essentially a civil war between all sides, like Bosnia 1992-5, but much much worse. It was brother against brother in many instances. Germans stepped in when their interests (communication routes, possible landing for allies, propping up their proxies) were threatened. As Germans weakened elsewhere, partizans finally conquered with Soviet support and communists ruled after the war and all lived happily in brotherly love till late 80's.