The US and the UK would have had a difficult time defeating Hitler without the USSR,
More of case of USSR didn't really need much of help from US/UK who launched their invasion mostly to prevent USSR from taking pretty much entire Europe under it's wing...Got to prevent USSR from going all the way to Atlantic and saying "haha German and France are now part of USSR!".
Always amuses me to read those 3 being called as allies when it was more of having common enemy to fight...For NOW.
As it was without US+Britain having logistical issues and USSR being more than a bit worried about the atomic bomb there might have been US+Britain vs USSR all-out war on Europe right after defeating German.
Yes and no. In 1943 the USSR really felt that they had their backs to the wall - a lot of their industrial areas had been overrun and they needed time to move what they'd saved eastwards and rebuild their forces. This is one reason why the US/Britain/Commonwealth invaded Italy - to put pressure on the Germans and force them to divert a lot of their troops away from the USSR, giving the USSR the time they needed. The USSR would probably have survived, but would have lost a great deal more territory and would have had a much tougher time forcing the Germans back. If the USSR hadn't been fighting the Germans quite so hard in 1944, D-Day would have either never taken place, or it would have failed.
'Operation Unthinkable' was the codename given to the plan to invade the USSR in 1946 - Churchill was keen on it, the British general staff felt that the USSR's 3-1 superiority was too great (even allowing for the rebuilt Wehrmacht fighting for the allies), and a lot of the US generals (apart from Patton) weren't keen on the idea. I've no doubt that if we had fought against the USSR in 1946, we would have needed to use nukes against them, but it wouldn't have been quite like Japan - the industries in the USSR are dispersed, with huge areas between - a few nuclear strikes on the USSR wouldn't have had the same political impact.