My sense of the game is that, between players of equal ability, the Germans can pick one objective (Moscow, Leningrad, or Rostov) and be pretty sure of getting it. Two objectives is a stretch. Three is very unlikely.
But to the original post's question: I think the Germans decided not to push to take Leningrad once they realized that they could not cut it off. I think the trick of bombing Osinovets is a gamey tactic, not a historical possibility. The port damage and repair rules are probably going to be revised in the next patch to address the equally gamey tactic of getting Odessa back on line one turn after capture and advancing the Axis supply lines in the south, and this will probably have a similar effect on the Osinovets gambit. So realistically, without that, you have to sweep the Lake Ladoga shore. Sviritsa is really hard to take. The Volkhov is a major river, and then there are two more rivers and a bunch of swamps to go through.
Another thing this game ought to do is make Leningrad and other major city hexes in the USSR into supply sources (maybe only if they have armaments factories present). Realistically, as you point out, Leningrad would not have had a shortage of ammunition or capacity to make/repair weapons. Currently, the manpower produced in isolated cities cannot be used to replace losses in those cities' garrisons due to lack of armaments, and the troops in those cities suffer isolated penalties. Properly simulating the intense urban combat that took place in these cities - one of the distinguishing features of this war and the main reason that the Germans did not try to fight their way into Leningrad - means treating Leningrad as in supply and able to produce replacement elements for its garrison as long as it has armaments factories. This would probably require a substantial code change, though, so I'm not counting on any changes.
This would also be an issue if the Germans manage to isolate Moscow. With Moscow's huge population, substantial industry, and location in the middle of a big agricultural region, I can't imagine that troops there would die from attrition or surrender because they were "isolated". I would expect them to fight to the end.
This is a feature of World in Flames. Any home nation city is a supply source, so Russians in Leningrad (or any other cut-off city) are always in supply.
< Message edited by thedoctorking -- 10/26/2018 8:25:24 PM >