Glantz believes that there are three decisive moments which can be used to define the war:
1. Moscow, which proved that Hitler could not win on his terms (IE, total conquest of the Soviet Union).
2. Stalingrad, which proved that Germany could not win on any terms.
3. Kursk, which proved that Germany's total defeat was certain; the only question remaining was how long and how costly it would be.
If you look at his book on Barbarossa and two volume Smolensk book he makes it clear that the Barbarossa plan was doomed to defeat from the start; its very premise was flawed, and Germany faced insurmountable manpower and logistics problems. Likewise, his two books currently out on Stalingrad demonstrate the absurdity of Blau; Germany attempted to attack along three strategic axis (Voronezh, Stalingrad, Caucasus) with a single army group, without the logistics to adequately support such enormous advances. While you can certainly draw the conclusion that Soviet victory wasn't inevitable, it's very hard to see how Soviet defeat would happen without extensive PODs which would radically change the entire war.
Define what you mean by Soviet defeat. Hitler's plan AA line was not feasible, but something along the lines of a Brest-Litovsk is not outside the realm of possibility...provided the West is not involved in the war.
Defeating Britain is also I believe a challenge that looks easy on paper but is impossible to achieve in reality. While the Luftwaffe was never truly defeated in its air campaign against Britain, it was slowly being worn down to the point where defeat would eventually occur; further, a sustained air campaign was impossible without denying resources to other fronts.
The only part of the air war that wore down the LW as the BoB; the Blitz actually saw LW strength increase, because the loss rate was so low until 1942 that strength was climbing, not decreasing, during the night bombing.
The earlier they start the night bombing and the longer it runs, the more Britain has its strength worn down due to lack of resources, making it increasingly hard to pivot away from daylight defense increase to night defenses.
What other front was there in 1940-41? ITTL the LW won't have a reason to get involved in Libya if they start bombing Liverpool in July, because the convoys that allowed the British to launch Op. Compass originated from Liverpool; being bottled up by air deployed naval mines in the Mersey, while port facilities are being degraded by night bombing seriously hampers the British ability to unload/load convoys once France falls, because of the port areas in the West, the only area 'safe' from German naval units and close LW bases, have limited areas to handle the necessary tonnage.
You've suggested alternative options in other threads, port bombing being the most interesting. However, your basic premise assumes that Britain will be unable to adequately counter German night bombing against ports merely because it failed to do so IOTL.
Also because once its enacted it becomes much much harder for the British to import the necessary raw materials and machine tools it needs to make the defenses that are necessary to defend the ports. Also there is the time factor; AI radar cannot be moved up any sooner than it was available IOTL, which means effective units are not available until March-April 1941 and then only in limited numbers. Even the Beaufighter needed time to get operational, otherwise rushing it means accidents and problems, delaying its effective usage. Also getting the necessary machine tools and raw materials for its require getting them into the country, which is disrupted by port bombing. Its kind of hard to start making them without imported aluminum or special purpose machines (of course there are general purpose machine tools, but that requires a lot of training to introduce a totally new type of aircraft and reduces output, pushing back their introduction vis-a-vis OTL). Then there is opportunity cost; what is Britain not producing to make night defenses? With less imports thanks to the mining/bombing of Liverpool and other ports there is much less material incoming than IOTL, so rather than just having OTL's resources to choose between, Britain has to cut across the board AND then make a choice about what gets cut to supply night defense projects.
But a sustained German port bombing campaign, if it developed into a serious threat, would see a concurrent increased British effort to defeat it. Not only that, but the British would engage it through indirect means by building up port facilities elsewhere to reduce the shipping bottlenecks that existed IOTL.
Where are the resources coming from to build up other ports? How long would it take? AFAIK building up ports with modern facilities is a process of years, rather than months. Then there is the issue of building up sufficient rail connections between these other ports and the rest of Britain; one of the major problems of the Western ports as that there were only a few that had the necessary rail connection to support the necessary volume of imports needed (Merseyside, Clydeside, and Bristol/Avonmouth; the later two only accounted for less than 15% of import capacity, meaning if Liverpool goes down, then the others could not pick up the slack). So any build up of other ports requires the build up of rail infrastructure. Not only that, but it requires the right port conditions (tides, natural harbor conditions, distance from LW bases). This would be a process of years, which would at the same time drain resources from the war effort; opportunity cost again. What can Britain spare when its imports are being smashed, limiting the ability of Britain to fight? What doesn't get produced to spend years building up alternate rail and port infrastructure?
American support for Britain would also increase enormously if Britain seemed worse off than IOTL; Roosevelt was by 1940 fully committed to US involvement in the European War, and would not accept the possibility of British defeat.
That is another assumption that doesn't necessarily hold water. IOTL what was US aid prior to May 1941? In September 1940 there were the 50 destroyers, all of which were nearly useless and required between 9-12 months to even get into service. Lend-Lease didn't pass Congress until March 1941 and only after it was apparent that the Germans weren't going to defeat the British via bombing (US Congressmen were very leary of giving Britain anything until it was confirmed the British weren't going to lose and turn it over to the Germans at the peace deal). Actually Lend-Lease didn't start until May 1941 though, which meant that the British were still paying cash for imports, by this time using Belgian loans, as Britain had run out of hard currency in January 1941.
Roosevelt may have been fully committed to keeping Britain in the war, but he couldn't get the votes to do much until March 1941, once the worst of the Blitz had passed. The US public didn't want to get involved in the war in Europe in the majority until some time in 1941. Here if the British looked like they were going to be in for serious trouble right after France falls, then there is opposition in Congress to supporting the British for fear of whatever materials given ending up in German hands. Without the BoB to rally the US public to the cause of Britain, there isn't going to be the same sort of support.
The BoB was the Western Front 'Moscow Moment' when it was clear the Germans weren't going to take Britain out of the war quickly. It was Hitler's first major defeat and showed the US that Britain was worth supporting. Without that moment, rather a start to a blockade, then the US public doesn't have the romantic story of US pilots helping the British in their hour of need, nor do they get the story of the evil Germans bombing British civilians in terror raids (Liverpool is a legitimate military target, as was Hamburg when the British started trying to bomb it in 1939). Especially if the import situation gets serious, rather than the US public being willing to support Britain, they instead will look at it as a lost cause and resource drain when the American military needs all of the US's resources to build itself up. IOTL Britain was viewed as a reliable shield to give the US time to build up it military and as a future base against the Germans, but that was only because the British had proven they could survive on their own; here that's not the case, as they wouldn't have a 'shining moment' to show they could beat the LW in a standup fight. Rather the LW would be able to bomb and mine the British lifeline with impunity (especially in 1940), which would result in serious damage to their ability to resist. So they end up looking like a lost cause, rather than a cause worth supporting.