The question and the first post have a slight incongruency. The real question for the scenario in your first post is probably something more like…. is it wise for the allies to go all-in in the Pacific, only to lose the game in Europe?
The main advantage I see to the allies breaking the the lend lease to land in Russia in the Pacific side is the distance between Hawaii/Guam/Philippines and Russia. American bombers can be produced one turn, in Hawaii the next turn, and then fighting in SZ6 and landing in Russia the next turn. Without landing in Russia, those bombers would have to wait one additional turn to rebase to Guam, so they can take off from Guam, fight in SZ6, then go land back in Guam. But once in Guam, all they can do is fight in SZ6 again next turn. American bombers in Amur, Siberia, or Soviet Far East could start doing strategic bombing raids on Tokyo.
Additionally, using Guam as a base allows British & ANZAC fighters & tactical bombers to rebase to Guam, then go fight in SZ6 followed by landing in Russia. America doesn’t need to use this tactic so much because they are likely to bringing carriers, allowing their small planes to mostly land wherever, however ANZAC & UKPac are unlikely to be burning money on carriers, so being able to land planes in Russia can be useful for attacks on the Japanese navy in SZ6 or even on Japanese land units in Korea.
Consider the case where Japan has left SZ6 particularly exposed, but Tokyo itself is pretty well defended with plenty of AA, and infantry, making a mainland attack not really an option… but Japan overlooked defending Korea so heavily. America’s navy doesn’t like its odds against what DOES exist of the Japanese navy in SZ6. They’re in America’s favor, but America would have to risk transports ready to capture Korea. But America has 3 strategic bombers in Hawaii that could really swing the advantage in America’s favor. They can’t land in Korea because America will just be capturing it this turn, so Russia gives America permission. With the help of the strategic bombers, which will ultimately land in Russia, America wipes out Japan’s SZ6 navy (barely… nothing is left of either navy in the sea zone) and captures Korea. On ANZAC’s turn, they fly 3 fighters from Guam to Korea. On Russia’s turn, they move 6 infantry & 2 AA into Korea. Is stuck and doesn’t have great options on their turn, and they’re particularly mad because they know on America’s next turn, Korea will have a mIC, naval base, and air base, and an American aircraft carrier with two fighters is in Hawaii ready to turn up in Korea to reinforce further.
… buuuuuut… if all this is happening while Moscow is falling, it’s not so great. It doesn’t impact UK & ANZAC options. ANZAC may still have built a strategic bomber or two somehow, and would like to base them in Russia in order to do regular bombing runs on Tokyo. But if America is spending all of its money in the Pacific allowing Germany to steamroll through western Russia, then it is probably America’s purchase choices that lost the game, not Russia losing out on 5 ipc per turn. 5 ipc per turn isn’t going to make the difference between stopping Germany or not from taking Moscow if the allies aren’t helping you anyway.
And the German player can easily shut down that bonus anyway by simply parking a submarine in SZ125. Germany should have cleared out the UK home fleet on turn 1, leaving UK with nothing in range for several turns (and probably more important things to do with its navy anyway). The Soviet Union doesn’t have a destroyer, and purchasing a destroyer instead of almost 3 more infantry is probably losing for Russia anyway (you will be spending 8ipc to hopefully kill a sub to get a 5ipc bonus back…and your destroyer won’t do much else… Germany can just ignore it. It only takes 2 turns after the sub is dead to pay off the destroyer, but you’ve gone one turn with 8ipc less worth of units that could defend Moscow). The only way America is going out of their way to kill that sub is if they’re planning on taking Norway and building a factory there… which is an option… but then you’re winning the game because America has a European factory, not because Russia is getting 5 more ipc. A lot of those America units are probably walking through Russia killing your bonus anyway.
Really though… Russia would probably like the UK to kill its bonus by lending some actual airplanes worth 10, 11, 12 ipc to help defend and possibly strafe good targets from Russia.