Second - FDR at that time was disliked a lot, much like good ol' Honest Abe was during the Civil War. But since they are both credited with so much, people of course change their tune. And let us not forget, the Great Depression would have never ended as soon as it did without WWII. SO giving him credit for ending it would be giving him TOO much credit.
Are you sure you are not confusing peoples' normal complaining with genuine dislike? They voted the guy into office 4 times. Did the republicans really managed to nominate such a noxious candidate each and every one of these races, or did people think FDR was good at his job?
Third - Again, we go back to the issue of it is not the majority who elects the president, it is the electoral voters who elect the president.
FDR won the popular vote in every election he ran in. In fact, his margin actually increased in the second election he ran in. It fell afterward, but never below a majority. Secondly, the way the electoral college works ensures that the popular vote decides the election on a state level. We have not yet had a significant instance of elector defection.
Fourth - WWII was not a very popular war with the folks at home, talk to someone who was old enough to remember, and they'll give you some stories you don't read in history books. Meatless Mondays, try meatless weeks for a many middle and lower class familes. A lot of people hated it, but they also saw it as a necessary evil that must be done to avenge our fallen brothers at Pearl Harbor and to stop Germany's aggression. Also, last I checked, the Iraq war, despite being very short, was never a popular war. What is going on over there now is a conflict.
I sat "popular" in the sense that the population supported the war, not in the sense that people were celebrating in the streets that fighting had broken out across the world. In that respect, WWII was a popular war. People generally thought that the US had a just cause and needed to go to war, as you said. When the US first invaded Iraq, a majority of the country favored the action.
Please, stop reading a standardized US history book. Because everything you've used to support your evidence is found in that. Go out and TALK to people who lived during that time. I spent quite a bit of time with my Grandfather a few years ago, and spent a lot of time hanging out in the local VFW talking to these guys who ACTUALLY lived during this time. I do believe that someone who lived then would be a more accurate reflection than what is in a book.
I just plain think you are wrong. Talking to people who lived is important for the facts it uncovers and for a more immersive sense of history, but records and statistics are infinitely more reliable than eyewitnesses. This has continued to plague modern day criminal courts. A person's view of their life is necessarily self centric, and memories are changed, consciously and unconsciously, over time. While certainly enriching, your grandfather's cannot be extended to a general trend.
Think about it this way: what is the likelihood that your grandfather and the locals he hung out and talked were a completely representative sample of the nation at that point? Statistically zero. What you are seeing is a small cross-section of a small non-representative group of people in the US at the time. You could make very compelling and accurate claims about these individuals or a group that you could show that they were representative of, but it does reliably not extend beyond that. People who write accepted academic history books generally use documentation and what statistics are available (and accurate, to the best of our knowledge) to draw conclusions because they are constant, and the correct statistics are known mathematically to be able to be expanded to a larger group with considerable accuracy.
On a note, I have talked to my great uncle about WWII, and he told me stories about his life in school at the time. It painted a picture of the times and was certainly helpful in establishing a background, but I would not claim to take his experience as representative historical fact concerning the entire nation.