I tried v5 as just the UK, allies as AI. This is surprisingly hard, and I'm concerned its overly dependent on dice/ AI decisions.
First try - lost Moscow turn 04. Uk could not put fleet down turn 01. AI USA did nothing. Germany got great dice, I felt powerless
Second try - Russia lost Karelia and Caucasus turn 01 so I just restarted.
Third try - Russia held out for a while. I played extremely aggressive with UK and got great dice, but it wasn't enough to win. I had to sack India to hold Moscow. The USA never engaged an axis unit until turn 04 when it lost its navy to Japan.
This game made me feel this was not a good choice for the challenge, after I took so many risks, they all paid off, but I still had no chance. For example, turn 01 the UK attacked Japan's East Indies fleet and took only 1 casualty (an extremely good outcome for Allies), which should put Japan in a terrible situation. But I just didn't have the production capacity to pressure both Axis players. I think the issue is that the AI USA isn't that great, while the other nations AI is pretty good.
Fourth time I let myself be both the USA and UK, with my Russian ally as the AI. Won pretty easily. This might be a better challenge, with an Axis bid added. Any bonus directly to the AI will benefit Russia as well, but still might be worthwhile. This challenge also makes a lot of sense.