Nah, I doubt the West would allow Germany swallow anything they want in any scenario, and Poland would definitely fight (even if they lose badly).
Meh, with Hitler they only eventually drew a line in the sand about Poland because he betrayed Munich agreement by invading Czechia first and he left very little room for compromise during the Polish crisis (he wanted war to conquer all of Poland and turn it into a Lebensraum playground, and at Munich he had felt cheated since he wanted to conquer all of Czechia).
Without the Nazi at helm, none of that happens, almost all non-Nazi German nationalists had little interest into annexing Czechia or Poland, they wanted Austria, Danzig & the Corridor & Upper Silesia, and the Sudetenland, in that rough order, at the very very most they would claimed Posen and made Czechia and/or Poland into independent satellites if a war occured. Such claims were wholly compatible with the interests of France and Britain in the 1930s, so the Western powers would keep on with the appeasement strategy and made a Munich II over Poland.
Poland may or may not fight, but if it does, it shall do so alone. Britain and France would apply diplomatic pressure to prevent the destruction of Poland when Germany wins. But again a nationalist Germany would press the 1914 borders as maximum war aim, and the Western powers would accept such a peace. That was the last real German irredentist claim in Europe, and afterwards, tensions between Germany and the Western powers would steadily diminish. The potential for limited wars between Italy (with Hungary and Bulgaria) and Yugoslavia, or between Hungary (with Bulgaria) and Romania definitely existed, but they are nowhere likely to draw the other great powers in their midst (well likely Russia may exploit the latter to grab Bessarabia).
Japan would probably create havoc as OTL.
True, however, without Germany or Russia pinning most of Franco-British power in Europe it would face a failure even more abject than OTL.
Stalin... only with a aggressive Germany would he dare to make a move. If Western Europe are united, Stalin wouldn't be able to conuer anything, and he knew that.
Stalin was less megalomanic and a gambler than Hitler, but to assume he would never start a WWII is foolhardy. The man was prone to make bad lapses of judgement about other powers' willingness and ability to fight (Barbarossa, the Korean War, Winter War) and to see threats where none existed, as he was more paranoid than Hitler. As such, it is perfectly feasible that he starts a general war because he mistakenly thinks the Western powers are too weak or unwilling to fight, or because he thinks they are going to attack him anyway, so he goes for a pre-emptive attack. Also, one must never forget that according to his own plans, the USSR would have been ready to fight a general war in 1942-43.