Operation Barbarossa could have very well succeded if it had been planned right. If Hitler had not constantly switched targets. First Leningrad, then Moscow, then the Caucasus, then Stalingrad, he could have defeated the Soviets. DOn’t forget that by Spring 1942 the situation for the Allies was looking very grim. The Japanese had swpet through Malaya, the Philippines, the Carolines, New Guinea, the Solomons, the Gilbert Islands, Hong Kong, Wake Island, the Dutch East Indies and Borneo Celebes. The USSR was on the verge of collapse since about 40% of it was already under German control and the ill equipped Soviets appeared to be about to fall. In Africa, Rommel was on the attack.
About Oepration Sea Lion, it would have been extremely hard to conduct such an operation. Remember that the Royal Navy was far superior to the Kriegsmarine throughout the war and that the best German alternative to transports which would have been torn apart by the RAF, Royal Navy and RCAF were paratroopers. But even that would have taken a huge toll on the German military. You have ot take into account the British anti aircraft fire would destroy some of the planes before they would be able to paradrop their troops and that paratroopers would be pararopping into unkwon enemy territory and could be easily ambushed. Also, it is important to remember that Spitfires coudl out maneuvre almsot every German fighter and bomber such as the Stuka Dive Bomber so the Germans would have had considerable trouble getting enough air support to defeat the British. America and Britain had already planned a Germany first strategy so I’m sure the Americans would have declard war if Operation Sea Lion had begun just in order not to be fighting alone against a triumphant Germany which would have caused more problems for the Germans.