2. No German declaration of war on the US. Once Pearl Harbor happens a countdown starts for direct American intervention in Europe, Hitler merely accelerated it by declaring war 5 days later. Let's say Hitler decides a war between the US and Japan is good for him and distracts the US. While intervention was polling high by this time there would still need to be something to trigger a direct US intervention. Without the declaration of war by Germany they could buy themselves a few more months before a US declaration.
Thing is though, with the US at war with Japan, US military production is going into overdrive. For the war in Europe, the US themselves couldn't contribute much until Torch happened. The US will pour resources into the USSR and UK regardless if they're at war with Germany (lend-lease with the USSR already started before Pearl Harbor). Which in the end will mean there's barely a delay with US operations in Europe.
So even if Hitler doesn't declare war with the US, it won't buy him much time. In fact it may cost him time, because it butterflies Operation Drumbeat, and it gives the chance to get the US their ASW together before they join the war with Germany.* Meanwhile I'm pretty sure the US starts escorting convoys all the way to the UK. So it's possible the tide against Germany in the Atlantic will turn sooner than OTL. At least they'll miss most of the second happy time.
And as
@CaptainCalvert posts, Stalin (and other Russians) won't be inclined to believe Hitler who already had been on record wanting Lebensraum and to enslave the Russians.
* May be a bit doubtful though. However it's important to note that one of the reasons of the slow response to Operation Drumbeat was a lack of escorts, which is one of the reasons convoys weren't deployed early on. There were also other reasons (for instance not wanting to dim the lights of coastal towns, King's hesistance to take the RN's advice).