1) Dunkirk was a great morale boost, but it's loss would not have lead to any sort of British capitulation. The lack of a morale boost is not the same as a hit to morale. And after the fall of all of France there really wasn't much of a greater morale hit to be taken at that point.
Of course the British would not have capitulated, but it would not have been good for British morale if the Germans would have killed or captured the whole force in Dunkirk, instead the Brits got a morale boost from this, when they escaped from Dunkirk
2) The Germans really weren't all that interested in North Africa and never committed much to it, nor do I think they could have/would have in May 1940. They had just conquered France and were looking to England. Even when they did "care" all you got was the Afrika Corps, which was not that much of a commitment.
Yes, I agree, but I think this was a great mistake made by Hitler and the other German military leaders. They should have sent a bigger force to Africa in the late summer of 1940, and driven the Brits out of North Africa.
3) Barbarossa could only have been started so much sooner. The rasputitza (mud season when the roads become impassable) existed in the spring as well as the fall. And this would only have helped if you think that it was the winter that stopped the Germans, and not the fSoviet ability to mobilize so many more men as fast as they did. Or that defeat of the Russians necessitated a supply chain far in excess of what was expected to be needed. As one Russian general said about the theory that the winter was so aweful to the Germans, "we had to fight in it too".
Of course the weather was the same for both, but the Russians had better winter equipment and were better prepared for the winter. I also think the Germans would have advanced further if they would have started the Operation Barbarossa earlier. I do not know how fast they advanced per day or week, but if they would have started 5-6 weeks earlier, then they would have advanced so much further.
The Germans executed Barbarossa essentially as they planned it. it probably could not have gone any better. But they failed not because of the timing of Barbarossa but because they didn't understand how resilient their enemy was going to be. The Soviet Union had 14 million men with military experience who didn't show up in rolls of their strength.
So many of these theories are ask about how much better the Germans could have done it. But you don't get to have perfection No one seems to theorycraft the allied side. Zhukov could have done a more focused counter attack in Dec. of '41 instead of the broader front approach he used, and it would probably have destroyed Army Group Center already in 1941. Had Stalin permitted retreats more units would have remained operational through the fall of 1941. There are lots of "what ifs" on the allied side as well.
The most German leaders were of course too optimistic and were not completely aware of the long distances in Sovjet Union, which is a far bigger country than France. But the fact is that they advanced in Russia until the winter come, and probably they would have advanced further if they have started the attack earlier. How much better it would have been, we can never know, but better for sure.