a) Germany uses more divisions from 15th, 1st, and 19th army areas earlier, held back OTL.
Which ones? Germany had a lot of rear echelon units held in France. But does their combat capacity match a frontline crysis like the one that Avranches' breakthrough represented?
b) Germans correctly anticipate Soviet intentions for Bagration and the Soviet offensive is less successful. More reserves are available.
Oh but they anticipated such offensive. It was lack of air recon that kept them blind about where to commit their reserves. In fact, by summer 1944, army groups in the eastern front were strengthened to their highest watermark regarding men, guns and tanks. But Luftwaffe's presence declined so decisevely because of Defence of the Reich campaign that suck strengthening of the eastern front became relative.
c) Germans evacuate 17th army earlier from Crimea creating reserves just now ready after refitting.
If an evacuation of 17th Army was to be done earlier, it would had to be by august 1943. It would have freed reserves for Manstein in the Dniepr crysis, but would have also freed reserves for soviet exploitation. It is know that Manstein was a brilliant tactician and probably much more wiser than Vatutin regarding modern warfare. But in any case, a real possitive effect of evacuation of 17th Army could only be mastered if Hitler gave up Heer command and let Manstein act on his own initiative. Manstein knew the danger of letting the soviet armies reinforce their bridgeheads around Kiev and Kanev. He already had presented a plan to switch forces from south to north to Hitler and go for a second 'Backhand bow'. 17th Army wasn't a very good army indeed. Despite it's german component and some romanian elite brigades it had plenty of 2nd and 3rd class romanian troops. That kind of troops weren't suited for eastern front. And if deployed in Normandy... Well they can be used as rear echelon reserves, but you can't expect them to fight very consistently, neither Antonescu would have allowed them to be deployed in France.
d) More rational defensive policy in the east September 42-June 44 just has more reserves available that can be sent east to west.
Again: this implied Hitler giving up command of the Heer. If that was the case, von Brauchistch was mentally exausted after Barbarossa. So any commander would have been picked by Hitler anyways, and would be an obsequent one. But as this is a what if, let's say that by miracle, Hitler picked a modern warfare battle hardened commander like Rundstedt, Guderian or Manstein (not necessarily one of them, just skilled enough). In that case, the 1942 campaign wouldn't probably ended in the loss of Germany's best sons: the 6th Army. 6th Army represented a lethal loss for Heer. The entire post Barbarossa German strategy depended on combat formations 'capable of withstanding a crysis'.
What is crysis withstanding capability' in eastern front terms? According to Earl F. Ziemke it's the ability of german formations to hang tough against daily artillery barrages, keep cohesion under multiple attacks and remain both moraly and disciplinary strong even if surrounded by large concentration of enemy forces. 6th army was that: the best german drafts of the lastest ten years were there.
Your last point is very interesting because if the germans kept avoiding their armies to be encircled they would have preserved a great potential for bold counter-attacks and possible new large encirclements of soviet armies. History shown us that even in the desperate days of fall 1943, german panzer divisions were still capable of piercing through soviet formations and attempting 'kessel' formations.
But then there's another issue to your what if: casualty rate in elastic defense was high even if it was lesser than in static or stubborn defense. If the line was to be held, then Germany would keep losing his sons. The key was in returning to Barbarossa 1st objective: destruction of the bulk of the Soviet Army. If by a continuation of smart maneuvers, the germans could continue pocketing soviet formations (perhaps not as large as they did in 1941, but pockets as the Don pocket in fall blau account 50,000 men) and destroying them by remaining caplable of counter-attacking, then the soviets would eventually run out of reserves. If such was accomplished by fall 1943, Germany could have spared their 'crysis withstanding capable' battle formations to face Operation Overlord.
No. Not in 1944. The firepower the Allies were grinding with would blast a path, like the carpet bombing of Panzer Lehr that wrecked the division:
https://en.wikipedia.org/wiki/Operation_Cobra#Main_attack_and_breakthrough_25.E2.80.9327_July
This quote comes handy: by 1944 the composition of elite german units was diverse. He could be a 4-5 seasoned veteran, or a boy. 4-5 seasoned veteran psichology was bullet-proof. He was way beyond fearing death. He had seen everything. He knew what to do in any combat situation and he was ruthless, merciless and inmune to to any kind personal or sentimental loss (eg: a friend succumbing in combat or news of a raid killing his family back home). The boy was anxious, eager to prove himself before the veterans, he had received a tough training but also a brainwash about nazi fanatism. The boys of 12th PanzerGrenadier Division 'Hitlerjugend' are a good example of this.
If Germany deployed this kind of troops in France, a victory in any phase (dissembark, bocage or inland) is probably secure for them because their men would have been ralentless killing machines. I want to leave clear that by no means I'm talking about a 'superhuman' advantage on the german side. But the mind of the german veteran was way beyond the american or brittish veteran mind in terms of ruthless warfare.
But such deployment, exposed to allied air power... Well, it would have been similar to sacrifying 6th Army in Stalingrad. So bravo por your example wiking.
Do you think there is a reasonable scenario that isn't OTL 1944 that could see the bocage being held?
If germans had defeated or agreed terms with Soviet Union and could spare entire Luftwaffe for Defence of the Reich it is still doubtfull they can hold the bocage. Everything depended on it. Heer had really good battle formations on the eastern front, but deploying them right below allied bombers was madness.
Indefinitely? Not with the minor changes proposed. They can delay the breakout a while.
But let's imagine they do stop (or more likely, unbearably slow down) the Allies up there. That might seem to be "indefinitely"... but it's actually for 15 days after Cobra, and some 40 days after Overlord. Because on August 15 there's Dragoon. That will put things in motion, or accelerate them, up North too.
If the germans have either dealt or negociated with Soviet Union by fall 1943, and again had at least enough interceptors to force a 'fair' battle in the skyes... Well any crack battle formation from the eastern front would have eaten Dragoon for breakfast. Allies already had commited their best units in Normandy, so the quality of Dragoon invaders would have not been the best.
With a 1 month delay:
a) Dragoon would be interesting with the Americans still in Normandy. Within a few days the Allies are driving up the Rhone, would almost force a pull out of western France to the Seine. Would sort of change the historical viewpoint of the operation to a battle winning clever necessity, we meant to do that all along, hold the Germans in Normandy and then..., sort of thing vs. the OTL "why did you do that instead of the Adriatic" afterthought, since the Germans were already beaten.
After August 15th I would figure the Germans would want to try to stick a strong blocking Force in the Rhone valley and hold the line of the Seine. Being forced to withdraw from western France like this, one would think involve much less losses than OTL.
b) a longer V1 attack time period, probably only 2-4 weeks, maybe V2 bases held a little longer, Keeping Allies air power attacking ski sites and such longer would help the Germans. Little real effect but might add to historical and military interest of such weapons post war.
If allies retained air superiority, any scenario was doomed for germans, even deploying elite veterans from eastern front. In fact, the only thing that kept Heer alive in Soviet Union for three years was the capacity of Luftwaffe to contest VVS' air superiority and thus allowing germans to recon and plan accordingly their moves.