Moscow could have been hard, not impossible. It might well have been the easiest of the 'big three' cities (Moscow, Leningrad and Stalingrad).
With German production increased, more Soviet losses in the Winter War, no Axis losses in Greece or Yugoslavia, better equipped Germans, a handful of Soviet losses in Afghanistan plus Spanish, Greek, Swedish, Yugoslavian and as one person suggested possibly even Turkish troops adding to the assault, it adds up to a substantial difference. Will it bring about the fall of the USSR? Probably not, but the Axis will do better, far better.
No, it definitely would not be. Moscow is much better built than Stalingrad or Leningrad, with numerous large, modern building which are very difficult to destroy with artillery or vehicle fire. Moscow is also outside of the supply radius for German forces which Smolensk provides (Source; Absolute War by Chris Bellamy) meaning that any attacking force would run very low on supplies quickly. Moscow is also shielded to the north by the Moscow-Volga Canal, which present a major geographic obstacle to attacking forces. Furthermore the city was defended by 3 major defensive belts even before the city itself which could not easily be defeated by attacking German forces. German forces likewise would be very overextended in any push to Moscow which would leave their flanks vulnerable to concerted attacks by millions of Soviet troops and thousands of tanks and aircraft. Finally, winter is STILL A PROBLEM. Even with good uniforms the Germans lack the correct lubricant or fuel for their weapons and vehicles, which means that those effectively freeze up and stop working in the winter.