No, YOU DO NOT SOUND NAIVE AT ALL!! I think your cost saving strategy is excellent. To answer your basic questions: Yes, you can test out of large number of courses/classes. It is call CLEP Exam (not sure if this is a universal term at all Community College) and is primarily a Computer Exam. You are given equivalent College credits for most courses that you CLEP out of. Below is a link to a particular Community College in Florida and the courses and credit given.
http://www.broward.edu/admissions/testing/Documents/clepcredits.pdf
Additionally, when you enter “most” Community College, you are give an entrance exam, which allows you to place out of remedial Math and English Courses. For example, if your Entrance Exam scores are above a certain cutoff, you will be placed in College Algebra. Your SAT/ACT scores also allows you to be placed in Higher Level Math and English Classes.
https://clep.collegeboard.org/california-community-college-system-endorses-clep
http://www.csun.edu/testing/college-level-examination-program-clep
It appears you are from California, which has probably the lowest Tuition of any Community Colleges in the United States, at about $46 per credit hours. So, you can complete the first two years of College at an insanely low Tuition Rate. You can further reduce the time and cost by “Clepping” out of courses.
You can also do a General Associate of Arts or Science Degree without a major and allow a certain amount of leeway in the course selection based on your AA or AS Program.
One downside to going the Community College route is the limited availability of Merit Aid available to Transfer Students (not only from Community Colleges, but also other 4-year Universities).
Yes, you can take classes that do not count towards your degree. Just watch out for excess credit clauses that most Colleges/Universities have in place these days. Will not be an issue if you only do a few Classes for “fun” or expanded learning.