1. The prime reason for Japan to invade southeast Asia is to get oil supply. If Japan and Britain had allied, Japan would have focused its war on China. Britain had trained Japan's air force and helped with Japan's Navy.
Japan would still likely have to take the Dutch East Indies by force. And Britain's Alliance with the UK had ended in the 20s, when it chose to side with the US over Japan (before Hitler rose to power and well before the Germans quite aiding China militarily, in 1937).
With a common enemy, an ally between Japan and Britain could happen.
No it wouldn't. Japan wanted British assets, and the assets of its allies in Europe (France, Netherlands), and Europe is far more important than Asia.
Not to mention that the Brits were not going to oppose the US. And FDR was strongly opposed the Hitler (the more important war for the Brits), and was getting the US involved whenever he could.
2. Would the US join the war?
Yep, The US was going to war with Japan with or without Pearl Harbour, just a matter of time.
3. What would be the outcome?
No big difference.
Keep in mind that WWII was not one big war. It was two separate conflicts (with their roots well before 1937 and 1939). No matter what Germany did, Japan was going to war with China and the US (and therefore most likely Britain, France, and the Netherlands). US isolationism wouldn't change this because US territory was at the centre of the conflict (Philippines).
Meanwhile in Europe, same thing. Maybe Hitler doesn't declare war on the US, and that draws out the war, or even gets the Soviets the rest of Germany, and some other territory in the end, but once the US got going, FDR was directing it at Hitler, and before '44.