1. This site has some pretty detailed maps of the battle. It's only worth asking if you mean 'before the Russian counter-offensive'. In that case, the Germans reached the Volga has several different spots, but I suspect the commotion of the bloody street fighting meant little in the way of fixed positions; so it's impossible to say they reached the Volga "in the north" or south of the city. Stalingrad is strung out along the river, so it's not an insane amount of urban area to cut through to get to the river.

Once the counter-offensive got going, though, the Germans probably had the run of the west bank of the Volga at Stalingrad, because the Red Army no longer needed to hold them down in the city anymore. The Germans reached the Volga - and were trapped against it.

2. This is to do with the tactics of urban warfare. In fact, one could argue the Germans carried out too much aerial bombardment when it came to Stalingrad. Blasting apart buildings, it turned out, actually made cities easier to defend. The ruins of buildings just provide more hideouts for snipers, who are especially deadly and decisive in an urban battle. Snipers on both sides became the stuff of legend at Stalingrad. The German blitzkrieg methods didn't really take this into account initially, but they were quick to learn their lesson, which is why they tried to capture buildings like the grain silo and turn them into strongpoints in a more systematic approach to taking the city.

3. The Russian winter offensive of '41-'42 focused on the "center" of the frontline, pushing the Germans away from Moscow. As a result, there were insufficient forces in the Don sector to put up a viable resistance to the massive offensive the Germans had planned towards the Caucasus. I'm not as well-read as I could be on the subject, but from my understanding, the Soviets decided to build up a capable defensive force closer to the Volga instead of throwing their troops into the meat-grinder piecemeal. So, to answer your question, not much resistance - so little, in fact, that it probably got to Hitler's head.

4, 5, and 6 I am unable to answer.

2. They wanted to keep it intact for symbolic reasons, to show off to the German people that they really took it. It's the same thing with Leningrad. 4)Simply river embankments, of course they'd tried to stop the Soviets from reinforcing, the Soviets always managed to hold onto one chunk of land on the western side of the river at heavy cost.

I sense a serious interest in specifics here from a military historian viewpoint. In that case, I recommend David Glantz and House's trilogy on Stalingrad. It has all the answers you are looking for, in vivid detail. Three whole volumes on the lead-up and the battle itself. The one you'd find most interesting is probably 'Armageddon in Stalingrad', which is about the bulk of the initial fighting and then stalemate in Stalingrad, up to the point where there is pushback by the Red Army.

2)Why did the German command invest so many men in taking objectives such as the Grain silo and not just reduce it to rubble through air bombardment.

The Silo itself was seriously bombarded, but like many concrete structures, destroying the thing was easier said then done. Indeed, one of the big problems in the whole Battle (for both sides, but particularly the Germans) was that destroying structures often did not dislodge defenders, but only made what remained a well defended morass of debris even more difficult to assault.

6)Sub question not exactly pertaining to Stalingrad. What exactly was the "commissar" system in the Soviet Union? What was its point?

This was simply a political officer, and was a position abandoned by the time of Stalingrad (or shortly afterwards). They were designed originally to keep track of the officer class, but their interference was seen as an obvious detriment during the battles of 1941. By Stalingrad, their main role was reduced to one mostly of morale and indoctrination building, rather than a previous role of being basically the executive officer of a command.

Originally commissars were introduced in the Civil War to control traditional commanders, who often had to be recruited (or impressed) from the ranks of ex-Imperial officers and therefore were politically unreliable.

Their powers waned and waxed depending on the performance of the military and the political climate of the Soviet state, as /u/Spoonfeedme has already said.