Unlikely. While some more forces would have been available, there were simply not that many troops allocated to those regions in 1942, not when compared to the vast army which moved against the USSR. Looking at how things progresses, it would have taken significantly more mapower, and materiel to make any kind of serious difference on the Eastern Front. By the time huge forces WERE deployed in Italy/NA, the war in the east was already a lost cause, it just took a while for the Germans to realise it.

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Would the allies - without the distraction of North Africa/mediterranean/Southern Europe - have been able to embark on an invasion of western France in '43 to relieve Russia of pressure?

No. keep in mind the allies chose to go into Italy and the Med primarily because they realised an Atlantic landing was not feasable in 1943. Fllowing that decision, and under pressure from Churchill, who favoured the less risky Southern strategy, the allies went to the Med.
ps to defend Atlantic convoys.

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I am not sure what would have been the impact in the Pacific/Far east but perhaps Pearl Harbour may not have happened as this was based on the successful British attack air attack on the Italian navy at Salerno, which would not have happened if Italy had not entered the war!

Now THAT is an interesting idea I had not considered, you are correct that it was the attack against Italy (Taranto, not Salerno I believe) that demonstrated that arial torpedo attacks were possible against ships in a shallow harbour, and allowed the Japanes eto make the necessary modifications to their torpedoes. I wonder if, failing a reasonable chance at PH, the Japanese would have reverted to their original strategy of luring the US fleet across the pacific into a major confrontation? At that point, Battleships would have remained the staple of both fleets for several more years, dramatuically changing the face of naval war in the Pacific...