A. Tirpitz and Bismark weren't operational during the blitz. Had the others tried it they would more than likely have been sunk. The combination of mines, RN subs, RN surface vessels, RAF, and shore batteries would have been more than adequate to the task.

B. I believe there were extensive mine fields as well as shallows protecting Lenningrad. Cruising slowly off shore also offers a pretty good target for Soviet subs and aircraft.

C. If the KM moves all it's major surface units to the Med the British would have been estatic. They could move theirs to Gibralter and bottle up and eliminate them thier.

D. The convoys would for the most part have been able to avoid the "large group" while the British (and depending on date US) would have had a number of larger groups hunting it. The number of vessels combined with the speed of some of them would have meant finding and catching them would have been much easier. Range limitations on some of them would have also taxed Germany's at sea replenishment resources shortening the cruise(s).