So isn't the easiest POD then for Hitler to have simply not conquered Western Europe?
In my opinion, no. The key thing here is that Manstein in the post-war was allowed to give his primary account and paint this cozy narrative of him being the genius who was being ostracised by the army leadership, until he managed to get Hitler to see the brilliance of his plan. The truth is, there was good reason to oppose his plan if you start from a conventional military perspective. The Manstein Plan meant risking literally everything - to give one example, the entirety of Germany's armour was committed to that operation. Nothing in reserve. The Luftwaffe too managed to achieve localised superiority only because it was committed on an insanely narrow front. The sickle cut was going to either win you the war in the West in one stroke, or lose it in one stroke. If something had gone wrong... say, a traffic jam because a signals officer made a mistake. Or if the column had been spotted. Or if the Allies had kept armies in reserve south of the Ardennes, etc etc. Then Germany's war is lost there and then, the armour is gone, and it's just a question of how long can disorganised and demotivated infantry hold back the Allied advance through the Rhineland. In this scenario you don't get a longer-lasting M-R pact, it lasts as long as OTL and comes to a close due to a crushing German defeat rather than a crushing German invasion.
The only reason Hitler went along with this plan is that he knew Germany had no hope of winning the war conventionally, unless we intend "win" as "force a diplomatic conclusion that could be considered positive". Hitler wasn't seeking a redressing of borders or anything like that, he wanted to alter the global strategic balance. Which means he had to take insane gambles because if they pay off, then you win spectacularly, and if they don't pay off, then you lose - just like you would have lost with a more conventional military strategy. The conventional plan by the generals (a slog through the channel) was the uninspired repetition of what had already failed in WW1. So, to make a long story short: if the invasion of France fails, Germany is instantly doomed and so is the longer M-R pact you want.
No, if you want this to be a prolonged TL you need to walk a very fine line between having Germany do very well, and not have it do well enough that it can attack the Soviets immediately (or give it a different motivation not to).
I'm not actually sure the USSR would enter as an ally or co-belligerent to Hitler - but they very well might just sit the entire war out.
Well, they offered to in November 1940, as you can read in the draft treaty I linked. But that is beside the point - this is why my suggestion is to start with a POD in Operation Pike. If the West conducts an act of war against the USSR, or DOWs them, then the Soviets and Nazi Germany are both fighting the West in parallel, even if they are not allies. This is what makes them co-belligerents, especially with M-R still on. That might remove Hitler's incentive to attack the USSR while they are busy killing British soldiers for them...
... then again, he might also reason that he has to strike at them before they can actually carve out a new empire out of former British possessions. I don't know. But it seems to me like this is your best option: if they are both fighting the WAllies, then a compelling argument can be made for extending the Pact for the time being. Especially because there definitely were a few senior diplomats and party members within Nazi Germany, beyond just Schulenberg, who advocated for a totalitarian alliance stretching from Paris to Vladivostok (and Tokyo - Japan was an enthusiastic proponent of Soviet entry into the Axis). If every totalitarian country is fighting the same enemy, then their voice is strengthened. It might not be enough, but it's a place where you can begin.
my speculation repeatedly is that Germany needed a more
coherent KM/naval program, including large percentage of coal fired ships. this would allow them to build ships for the USSR (which against all reason they wanted)
the same could be said for their synthetic fuel program, worst of both worlds, they expended the resources but only finished during wartime , so no chance to reap the benefits or stockpile fuels.
hinted in earlier post they needed to occupy Romania rather than ally with them? (in agreement with Hungary and USSR) with Germany occupying the old
Kingdom
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this
might present a more dire scenario to the Allies and help provoke "Pike" (used as generic term here for any Allied DoW or operations against the USSR) but the objective is to place the oil under German control.
if they retained their Baltic trading bloc with exception of Poland (or rather half of Poland), a relatively robust synthetic fuels program, and had the Romanian oil in hand it would transform their economic situation vs. historical?
https://en.wikipedia.org/wiki/Nazi–Soviet_economic_relations_(1934–41)#Mid-1930s_deterioration_of_relations
who knows if the Soviets would agree to dealings that leave the Baltics and Finland independent?
The problem with different production lines is that you need a POD in at least 1933, and you have to somehow circumvent the tight corset the German economy is stuck in while trying to rearm. I really don't see a way around that, tbh.
Regarding the Baltics and Finland, I doubt it. As I mentioned earlier, Stalin's objective was the restoration of typical Great Russian chauvinist goals. Now with that said, originally Germany was meant to keep Lithuania, with the Soviets keeping a larger chunk of Poland. Berlin proposed to alter the border and trade Lithuania for a larger piece of Poland, and Moscow agreed. It's possible that if this doesn't happen, Germany's presence in the Baltics somewhat alters their economic situation. However, what you need to keep in mind re stuff like the oil fields, is that Germany constantly overestimated how much it could get out of its vassals/conquered territories, because they assumed they could just take over and production would continue at full capacity. That was almost never the case and it would take them years to bring those conquests up to speed. Especially in isolation from world trade.