In the event that Germany would have been winning in 1943, it would have been the USSR and not the US or the UK that the Luftwaffe would be facing.
In 1943 there was zero chance for Germany to defeat USSR. The Sovs had tipped the balance with respect to reserves, manufacturing and strategic resources. So the only chance for Germany to diengage would have been for either Hitler or Stalin (or both) to be dead and their successors negotiate an armed peace.
So, hypothetically say that happened - that would enable Germany to withdraw say 1/2 of their East facing forces but still plan a defense. In fact in mid 1943 Germany had already started deploying LW units from East and South to build LuftflotteReich from Mitte and by February 1945 moved more than 27 Staffel to Germany. The strategic positioning for raw materials defense would have been more assets at Ploesti available - but not necessarily deployed until August when Avalanche hit it.
The Russians were a type of people who would never give up.
I agree this.
Had Germany successful defeat the USSR in 1941 or 1942, it would have been very difficult for Germany to have control over Russia; riots or even a revolution may have occurred. Going back to 1943, if Germany won the Battle of Kursk, the Russians would have been set back, but not defeated. The reason for why it was such a huge turning point in WWII is because unlike Russia, Germany always had a lack of resources, equipment, etc...; the Germans could never recover from such a defeat.
As for the fighting against the US and GB, it was possible for the Luftwaffe to stop Allied Strategic Bombing Campaign.
It was possible to increase the casualties - But doubt that Germany could defeat the combined Allied Bomber Commands. Tactics would change, maybe 8th AF ceases operations over deep Germany until the Mustang and Lightning arrive in force - but the aircraft and pilot and crew replacement backlog was huge for the USAAF and FAR larger than the LW could match - much less train adequately even if Ploesti remained intact longer than summer 1944.
The strategic bombing campaign for the allies was a very difficult task to do. Aircraft needed to be organized and mobilized properly, bombers were costly and proper equipment and pilot training was essential. High casualties were all that was needed for the allied strategic bombing campaign to stop. Why didn't the Luftwaffe achieve this? Because they always had a shortage of pilots, aircraft, and equipment; the majority of all those things being used on the Eastern Front. It wasn't until early 1944 that the Luftwaffe began to mass produce their fighters, but by then, they didn't have enough pilots or fuel to use them.
In 1943 the majority of LW assets were in Wset, not east. By February 1944 it was 3:1 Germany/France/Holland to the assets on Ost front. In early 1944 the LW had exactly as much fuel as they had in 1943 with some reductions due to Ploesti bombing. If Russia/Germany do truce, the Luftwaffe doesn't have the oil fields of Ukraine and no sanctuary for training. The Germans could have halved their bomber force and shifted trained bomber pilots to fly fighters with perhaps less overall effectiveness - but better than the reality of mid to late 1944 fighter pilot replacement pools
If the Allied Strategic Bombing Campaign were to stop, the only place that Germany would need to fight the US or GB would be in the Mediterranean.
There is no scenarion for either or both Bomber Commands to simply 'stop'. There is also no tactical scenario that makes one believ that the LW would change its deployment strategy from East to West until the 8th AF showed it could not be defeated with even the mass re-deployment from East to West from mid 1943 to 1944. Maybe 8th AF has to shift to night bombing until long range escort arrives - with no reduction of strain to Luftwaffe where the targets were within range of P-47s for continued dalight bombing - but the 8th AF would revert back to daylight strategic bombing in January with the arrival of P-51 and continued build up of P-38s. P-38s were already escorting to Ploesti in September 1943.
I think from the beginning Germany and Italy had no chance of winning in N. Africa. Italy was not prepared what so ever and Germany always had a lack of equipment and resources. This was once again due to the Eastern Front. The only way for the Axis to have won was if Malta was invaded. This would have made it possible for enough equipment and resources to go to Tobruk which would have made the Axis forces win at the Battle of El Alemain.
By 1943, there was no way in which Germany would have been able to hold Tunisia; the only reason why it took so long for Italy to fall was that Germany from a geographical standpoint was at an advantage (the Gustav line). If Germany had more forces there i.e. not lose the Battle of Kursk, then I think it may have been possible for the Gustav line to never fall; a counter-attack may also have been possible.
Going back to the original question of could the Luftwaffe survive after 1943 if it faced only the US/UK airforces, my answer is yes. If the Allied Strategic Bombing Campaign was stopped (which was possible) then the only place in which the Luftwaffe would need to deal with US or GB air forces is in Italy. In Italy the Allies already achieved air supremacy yet achieving that had almost no effect on the fighting on the ground. Because of this, I would argue that it would Russian air power, which is almost limitless, that would have been the main threat.