Agreed, Hitler made some of the stupidest mistakes in the history of war fare. Attacking the Russians in the Winter, invading Poland even though he should have known the Britsh would declar war.
So If he didn't help Japan and the US went to war with Japan in the Pacific how would the US have gotten involved in the European war?
At some point Hitler was going to have to act against USN defended convoys. Once the Pacific War started I can easily see the U.S. escorting American and neutral flag shipping all the way to the UK (our ally now, at least against the untrustworthy "Japs") or even to Murmansk (the Soviet Far East Front is holding down a big chunk of the IJA, even though they are not formally at war). The USN would also be set loose to prosecute to kill any submarine contact in the Atlantic (again, those IJN boys are tricky, might be one of them there "Jap" subs after all).
Those steps would be easy for FDR to put into play. At that point the Reich can either end the Battle of the Atlantic, with all the dreadful prospects that carried in relation to British manufacturing and receipt of American/Commonwealth materials, or decide that they are going to take the gloves off and go after U.S. flagged and protected vessels in a serious way. If Germany backs down, the U.S. can now do the same thing into the Med, put an Army Group into India (the IJA is right there in Burma, just itchin' to get killed) which by coincidence, would free up 10-12 divisions of the Indian Army for Service in the Med, followed by U.S. troops taking over for our gallant allies against the Japanese in the Middle East, etc. Eventually the U.S. can move enough forces into areas that are plausable that the entire Commonwealth can toss everything in the envelope at the Germans.
So, at some point Hitler HAS to go after the U.S. or he winds up facing off with the entire Commonwealth and the USSR with absolutely no way to interdict their supply lines while his supply situation is strangled just as happened IOTL (and by the exact same vessels in many cases). That is a sure way to lose the war.