"1." doesn't really work out; given Adolf's ideological oddities, his Drang nach Osten fixation, overconfidence thanks to the success of the West Front blitzkrieg and the abysmal showing of the Soviets in the Winter War, plus the detail that fascism and communism were pretty much natural enemies, it's extremely difficult to see why he wouldn't have invaded the USSR the first chance he got.

I've read that Stalin is generally considered to have been of the "rationally unscrupulous" sort of tyrant, and had a deep dislike for risk-taking - if it was up to him, gobbling up half the East Europe and then coexisting with Die Reich was a right fine scenario. Alas, Hitler wasn't of the quite same mold...

Anyways, AFAIK Germany lost the Battle of Britain pretty decisively even without having had the better part of its resources stuck in the Russian steppes which had some major future implications.

"2." doesn't really work either - both Hitler and Stalin had too many practical reasons to give each other some breathing room at the time. But if one assumes they for one reason or another failed to give each other elbow room, it would seem that both would've had to keep rather larger garrison forces in East Europe to guard against a surprise attack from the other (that Germany *didn't* invade Poland is pretty much a scenario out of the question). Now, this would seem to have some larger implications. For one, the USSR might not have been willing to risk the Winter War, given that it would've been a very logical developement for the Finns to ask the Germans for aid; in this case they'd have failed to work out some of the major "bugs" in the Red Army, and conversely their poor showing wouldn't have made Hitler so overconfident. On the other hand a lot of German troops and other resources would've been tied down to guard the border against the Soviets, and hence unavailable against France and the UK - and that just might have been enough to bog the blitzkrieg down into trench warfare.

More importanly, however, if Stalin had no reason to believe Hitler would leave him alone, it's entirely possible he might've entered into an alliance with the Western Allieds instead - particularly if the fighting in France dragged out and drew enough German resources away from East Europe. Opportunism, after all, was just about his main operating principle...

"3." would depend on other circumstances, but if need be the USSR probably had quite enough military muscle to enforce its demands if necessary.

"4." would depend heavily on who succeeded ole Adolf. It is entirely possible the top military brass (or at least parts thereof) might've tried to pull a military coup, which just might've completely fractured the Reich into a civil war between the rebellious generals and Party loyalists. Now that would've been messy indeed...

In any case, if the rather more rational among the senior officers could garner enough influence it is entirely possible they'd have tried to negotiate surrender with the Western Allies, whom they for rather good reasons considered an evil far lesser than the understandably frighteningly angry Soviets. Which in turn might've ticked the more fervent Nazis off enough to start an uprising of their own, and/or might've led to direct conflict between the Western Allies occupying Germany (plus their new German subjects) and an USSR not content with the results.

Mind you, if one of the Nazi believers got the job and proved a more competent supreme leader than his precedessor, the Allies might've had to fight a bit longer before Germany collapsed completely; Adolf's kooky strategic ideas, after all, often ended up wasting German resources to their advantage. In hindsight the writing was proabably on the wall by that point already (Germany may well have actually lost the whole war as soon as Barbarossa stalled...), and given the sheer economics of modern industrial war and increasingly desperate German shortage of about everything it is hard to believe the end result could've been much different.

"5." I'm under the impression the US was quite happy to supply both the British and the Soviets with material aid even before they were at formal war with Germany, so if one assumes the war went otherwise as it did this would probably mean the Soviets would've had to grind the Germans to dust mostly by themselves - although the Brits could fight the Reich to a standstill, it is extremely dubious if they'd have had the resources to counterattack into Festnung Europe by themselves. Well, at least before the Soviet juggernaut squeezed the German tight enough that they'd have to essentially abandon the West and move anything even resembling combat troops to defend the Fatherland from the communist hordes. Once that happened the British (most likely by that point joined by the Americans, who weren't any strangers to opportunism either after all, who ought to have finished with the Japanese by that time; all the more so as the Brist would've been able to spare more attention to their Far East front) ought to have been able to overrun at least France, Italy and so on - depending on how desperate the German East Front was, it's perfectly possible their local garrison commanders would rather jump ship to the Western Allies than sink into the Red ocean with Germany proper.

"6." would've been a pretty nasty scenario, given that the Germans also developed ballistic missiles. On the other hand, here the key question is when - after all, the US, which could spare the resources far better than the embattled Germans, had to expend several years of solid labor by a major team of scientists in complete safety (which the Germans didn't have, given the sheer range of Allied heavy bombers) and ridiculous amounts of money just to produce two workable bombs in conditions far better than the Germans were in only a few years into the war. Even if they got the theory working right, the Germans would have been very hard pressed indeed to produce workable warheads if they'd had to start during the war proper - and even if they succeeded the loss of the V-2 launch sites to the Soviets might well have left them with little in the way of suitable carriers. And merely vaporizing some field divisions wouldn't quite have had the same effect as wiping London or Moscow off the map - the Soviets in particular wouldn't have cared much as far as sheer casualties were concerned.

'Course, if they'd have somehow managed to develop at least the working basics before the war...