Germany had to defeat the Allies at the Normandy beach head. It was certainly possible in certain circumstances: for instance IF the Germans had of moved their troops and aircraft there at least 2 weeks earlier then what happened at Omaha might have lead to failure for the alliers there and at other landing sites. Had they widely introduced the rewirable reflector for the enigma machine; UKWD, (Unkehr Walze D) in 1943 then allied cypher penetration of enigma would have collapsed and with it a huge allied advantage. UKWD was only used in small numbers from 1943 and hence made no difference; in fact it sometimes made things worse as opperators asked "Do you have UKWD" which provided a crib for a break in.
What strikes me is the lag in introduction of the Me 109G-10, Me 109K-4 and FW 190D-9 and D-12 was so late. These aircraft need to be moved forward some 6-9 months so that they are seeing service at the begining of 1944 (january) in serious numbers instead of the end. It is at this time that the P-51B is seeing its first service. The Me 109G-14AM and probably also the Me 109G-14ASM also need to be moved forward 9 months and the 1.42 ata rating also needs to be released earlier. This at least gives the Luftwaffe pilots aircraft that keeps more of them alive. At the very minimum the very basic mods that could be done to the Me 109 to keep its speed up need to be introduced in the manufacturing line:
cleaned up gun bulges, retractable tail wheel, ERLA haube, galland hood and higher precision manufacturing. The latter alone can easily add 12km/h. In total there is over 12 perhaps 20 mph there. This was all understood.
I'm also inclined to assert that had a lightweight ejection seat similar to that used on the He 162 been introduced on 109s, 190s and 262s in early 1944 that so many more pilots would have survived it would have kept the skill level of the Luftwaffe up dramatically.
The Me 262 could have been ready. Though seeing service trials in mid 1944 it wasn't untill late 1944 with the introducion of the Jumo 004B4 with hollow blades that the production engine became reasonably reliabe. An aspect of reliabillity that could have been much earlier was better fuel control, the so called beschleunigungs ventile (accelerator valve) that would have helped to keep the engines temperaturs stable and controlled. This poor fuel control was a problem with allied engines as well. In general the engineers involved regretted that not enough attention had been paid early on in introducing proper control; there was not leap in technology required, just an application of existing art. Measure or estimate the airflow rate accuratly and add fuel proportionatly such that the engine neither falmes out from fuel starvation nor burns through due to over supply.