There's a bit of a wild card in the Hitler assignation possibility. The Casablanca understanding was Roosevelt's idea with Churchill going along with reservations. The objective was to bind Stalin to the effort in view of efforts by Japan and Italy to broker a separate peace between the Soviets and Germany.
This I find very hard to believe. I agree that the initial concept was suggested by Rossevelt, but given that Casablanca was a conference in which the british almost totally dominated the discussions, and came away with virtually everything they were looking for, and Churchill had always strongly advocated the principal of "no surrendder, no negotiation" it seems very unlikely that the british were reluctant to the concept
Chapter I: Casablanca -- Beginning of an Era: January 1943
While the unconditional surrender ultimatum did consolidate internal support for Hitler, the generals on the Russian front continued their assignation efforts. General von Tresckow widely and successfully recruited officer support, including Kluge -who didn't participate but neither did he report the plotting. By the end of 1943 six serious attempts on Hitler's life were made, including the wine bomb on his plane that failed when the detonator exploded but the plastic explosive failed to follow on.
Most of the Germans officers didnt believe the Casaablanca ultiatum was serious. They believed that with Hitler gone, the western allies would join them in a crusade in the East. They didnt seem to realize that it was they, not Hitler, not the Russians, it was the germans themselves that were the enemy.
Getting on to my take on this, the generals recognized that the war was lost and were concerned about protecting Germany. Hitler likewise recognized that the war was lost and wanted to take Germany down with him, which he largely did though not to the extent ordered since his order were ignored.
In 1943 hitler had had the stuffing knocked out him at Stalingrad and Alamein/North Africa, but he and most generals were far from defeatist. People like Guderiann believed that with proper rebuilding the German Army could bounce back and defeat the Russians, and after that, the western allies. Consistently the German intelligence services predicted the Russians were at the end of their manpower tethers. After Kursk, the Germans consoled themselves that would be able to wear down the Russians by onesided attrition. they were also caught short a little, because they mis-judged the Russian capabilities (the Russians began attacking, and kept attacking summer or winter). unlike the German offensives, that had been punctuated by ling breaks during the winter, the Russians never let up on the Germans after Kursk. This caught the Germans a littler by surpise, and wore down their reserves at an unsustainable rate. They were never able to recover after Kursk, never allowed to.
Hitler also deluded himself, and a lot of other people that his new terror weapons would turn the corner for the Axis. The new technologies....Type XXI tanks, Me 262s, V1s, V2s, you name it were supposed to get back the initiative and halt the offensives against Germany. They did nothing of the sort, but that was not known in 1943....
Had the assignation been successful, the army would still have to deal with entrenched Nazis and the SS divisions, though there was an approach during 1942 that suggested even these groups were onboard. However, during 1943 a separate peace would have been attractive to Stalin as he would gain most of Eastern Europe, Casablanca be damned. Having split the allies, Germany would be in a rather strong position to treaty for Western Europe.
What is the basis of assuming that Stalin was amenable to a separate peace. There had been some talk of a separate peace immiediately after Stalingrad. After Kursk, no such feelers were ever put out. Stalin could be trusted to the extent that he was a barbarian driven by base emotion. His primary emotion after june 1941 was revenge. you could rely on that. Later, as victory became more certain, you can add greed to revenge. He wanted the control of Eastern Europe and meant to have it. This was a centuries old Russian objective.....
The peace feelers that were put out after Stalingrad were very low level, and really not intended to come to anything. The Russians made it very clear that they wanted full control of the Balkans and Poland to the dfemarcation line. this would have completely imasculated the Germans, as they would have to give up Rumanian oil. It simply was never going to happen, and Stalin knew it.
Getting rid of Hitler would have solved nothing. it simply would have made the germans more isolated, after they realized they faced total defeat with or without him.