I think that people underplay exactly how much of OTL's intransigence on negotiations were based around the Allies have the whip hand by 1943. They assumed they'd be in a position to win completely without negotiating, so preferring to fight it out; however is they thought that there was a really good chance they couldn't win outright or that the cost would mostly fall on the Wallies in terms of lives, not the Soviets, they'd rethink that at some point. However the Hitler part would have to change, so likely they'd let the Germans know via Dulles in Switzerland that Hitler would have to go to get negotiations under way.
Sure, but IOTL by 1943 it was clear the Germans were going to lose, it was just a question of when. If the Soviets collapsed in 1942 and the Wallies would have to fight it to the better end themselves...they'd likely focus on regime change via back channels and try to negotiate with his replacement. And the thing is they'd likely have assumed that it would take MORE than French WW1 level casualties to win total victory.
IOTL they planned on murdering everyone anyway so why would they care about doing it directly? The whole 'just starve it out' was window dressing to his realization that he couldn't take it by storm in September without compromising Typhoon. And Leningrad was very important as a production facility and tied down huge Axis resources while also denying them use of the port for getting supplies to AG-North, while keeping the Soviet Baltic fleet alive and a threat, with their subs raiding German convoys to Sweden.