At the start of the war, sure. The Red Army was a hot mess still suffering from officer purges and outdated equipment. By the end of the war, the Red Army was very, very, very good.
Why were their flanks exposed? Lack of troops and resources. That they were forced to rely on increasingly unreliable troops to defend the flanks -despite knowing full well Soviet Deep Battle doctrine loved them some grand flanking maneuvers- just shows how over-extended the Nazis were when they tried to lunge for Leningrad, Moscow, and Stalingrad.
Again, real life warfare isn't a civ game. You can't just stack an arbitrarily large number of units in an area with no logistical issues. Armored units require an obscene amount of logistical support. Every additional soldier needs to be fed and equipped. Every additional vehicle needs gas, replacement parts, and replacement vehicles. Additional resources need to be set aside for non-combat repair formations. Almost all of this had to be carried thousands of miles by truck across the rutted cart tracks which were passed for Russia in roads. The different Russian railway gauge and partisan activity hampered Nazi efforts to use the Russian railway network to move troops and supplies. All of this placed additional limitations on an already overstretched Nazi logistical operation.
Yeah, their bodies were scattered across the Ukrainian plains.
Explain it to me like I'm 12. What were the Nazi's long-term goals in getting involved in propping up the Italian's North Africa debacle? They were able to force the British Commonwealth to avoid the Suez and send supplies around the Cape of Good Hope. While that was an inconvenience to the British, was hardly the end of the world for them and something they would already be doing until they controlled Sicily and the boot of Italy. The Nazis were occasionally able to attempt to threaten the Suez Canal, but due to supply problems never really were able to do so seriously. The Nazi's half-assed some attempts to start an Arab uprising, but those never really got off the ground.
All that getting involved in North Africa really did was put the already precarious Nazi logistic and supply abilities under additional stress. If controlling the Mediterranean was as important as you say, the Nazis made horrendous blunders by deciding to invade the Soviet Union. (Never mind that the Nazis were on the verge of bankruptcy to the Soviet Union and in desperation was selling them battleships and fully formed industrial plants capable of making synthetic rubber.)