von Hindenburg being President or Wilhelm being Kaiser changes little assuming a death - in both cases there's a process to get someone to replace them (new elections for President, coronation of the heir apparent). Hitler's popularity and power allowed him to skip the whole process and just assume the position of Parliament too. I think that it would have been harder to do with the position of Kaiser (there were still plenty of monarchists even in Nazi Germany), but he still would have done so or at least done a Horthy, preventing the coronation and keeping himself as regent.
There were actually numerous plots to assassinate him; the July 20th plot is just the most famous (probably because it came the closest to success).
This guy came very close just a few months after WWII started: https://en.m.wikipedia.org/wiki/Georg_Elser
Japan also had a monarch, and it took 2 nuclear bombs to get him to surrender.
After President von Hindenburg's death, there was no one left in Germany for Hitler to answer to, not even in theory. That meant there was no legal mechanism to remove him, he could only be removed by violence – and the plot to assassinate him failed. The world could easily have been a better place if Germany had remained a monarchy – even if Hitler still became Chancellor and started World War II and all the rest of his horrid crimes, it would have been far easier to remove and replace him with a new German leader. Germany's generals knew they had lost, and many of them wanted to focus on making sure they surrendered to the Western allies not the Soviets - but Hitler forced them to keep on fighting unwinnable battles instead. With Hitler imprisoned (or maybe even summarily executed to eliminate the risk that devout Nazis would try to free him), the generals could have done as they wanted. Possibly the division of Germany could have been avoided, and large swathes of Eastern Europe could have escaped communism. It also could have resulted in much better treatment of German civilians and POWs, for while the Western allies weren't perfect in that regard, they were significantly more humane than the Soviets were.