Like most movies, The Battle of the Bulge is set in a historical context with specific events that are made up. There actually was an important battle, called the Battle of the Bulge because of the salient it created (when one side breaks through enemy lines in a specific spot, creating a "bulge" in the line). The German plan was to use this salient to divide the American and British/Commonwealth forces, end their cooperation, and discourage them so much they'd stop the war.
The problem was that the German plan relied heavily on fast-moving armored units ("Panzer" being the German word for "armor" - often translated as "tank," it also applies to other armored fighting vehicles. For example, the Panzer IV is really short for Panzerkampfwagen IV, or "armored fighting vehicle #4.") German had chronic problems with petroleum - this was a major factor in its takeover of Romania, as well as its push into southern Russia towards the Caspian Sea. While the big clock on the wall was most likely just invented for the movie, there were certainly calculations by the Germans on how long their attack could last before it ran out of fuel. During the actual battle, many German armored units did run out of fuel and were forced to abandon vehicles. The Germans would certainly have tried to capture as much fuel from the Allies as possible, though the fuel depot in the movie doesn't represent a real, specific place.
To answer your specific points:
*If oil was poured down a hill and set on fire, the ground would burn, but it wouldn't create a bubble of fire around a tank - the oil on the ground would burn, some oil would stick the tread of tanks and burn while on there, but this wouldn't be as big of a problem as it seems in the movies. The tank would get really hot and the fuel tank would explode after awhile, but everyone should be able to get out of the tank first - running through the oil on the ground would be the biggest concern.
*The Germans had many bunkers throughout their occupied territory. Bunkers didn't play as extensive as a role in World War One's trench warfare along the front lines, but - just like today - bunkers are used as air-raid shelters and extensive ones are used for protecting important people. Keep in mind that the Germans invaded France in 1940 and the Allies didn't open the "Western Front" until 1944. This gave Germany plenty of time to build fortifications. Bunkers and underground complexes played an increasingly important role as the Allies gained air superiority around 1942-3, and air supremacy in late 1944. By the time of the Battle of the Bulge, the Allies controlled the skies - allowing them to conduct reconnaissance of anything above ground, and to bomb whatever targets they spotted.
*During World War Two, most tanks were composed of a hull with a ring that the turret was lowered onto - making them two separate parts. Many tanks lost their turrets, but for one to be "blown off," there would need to be an explosion in the tank that would almost certainly kill anyone inside. In theory, a shell hit on the side of a turret could "knock" it off, but this would require the turret to be loose - meaning there would have to be other problems as well. While I wouldn't say it's impossible, I don't see how anyone in the crew could survive. Also, many WW2 tanks were driven using overhead levers, which would likely be destroyed if the turret came off - meaning there's also a mechanical issue with the tank driving off.
BA Military Studies