PhD student here. In my experience, classes don't actually get that much harder than the upper level undergrad courses. Don't let that prevent you from pursuing a higher degree. The research and work outside of class for a PhD is much harder. I have also found that grad classes have easier grading systems. Obviously depends on the program. But in my experience, the professors understand that some of the classes you take for degree requirements have nothing to do with your research focus, and your research is more important. Instead of grading on a A-F scale they'll grade on an A-B. If you are in a PhD program they will do everything in their power to get you through classes so you can get to researching. If you struggle with the research aspect, then there are checks in place - candidacy exam for example - that will prevent you from getting the degree.

It completely depends on the program. One program I looked at focused on producing academics. They believed that if you got a PhD you should be able to teach any of the core classes in that field at any level. Obviously they put a lot of effort into ensuring the coursework was the focus. Even then though, so long as you survived the class and didn't completely screw up a project or exam, you would still get an A. Grades don't matter. That's not how you demonstrate your expertise in a field. The program I'm in currently focuses much more on the research and you can't learn that in the classroom.

Finally, nobody is going to care what your gpa is if you have a PhD. The degree completely speaks for itself. They will care about your dissertation and if it is good research. They will care about publications, conference presentations, grant awards, patents, etc.

Your undergrad record will get you into better programs with better scholarships. After that, you'll work your ass off for 4+ years and classes are just apart of the grind for the first year or two. But if you obsess over your gpa you will lose focus on the research and that's when you get into trouble. Classes are not what you should be worried about. Find what you want to do, decide if you really need a PhD, and go do it! If you have a good work ethic, and it seems like you do, you will be perfectly fine.