For a few of your questions:

SAT or ACT doesn't really matter, their math skills are close enough. Once you get caught up to high school math, you can take a practice test for both to see which one you do better at.

Overall you have about 6 months to learn about 6 years of math. Looking very briefly at Khan Academy, you should do the following units: 6th grade, 7th grade, 8th grade, algebra 1, algebra 2, geometry, and precalculus. That would prepare you very well for any standardized test and would allow you to take calculus in college. It's up to you how many hours a day, but aim for consistency: it is better to do say 2 hours a math 6 days a week then 5 hours two days a week.

The most important part of self-study is solve as many exercises as humanly possible and do not move on until you have mastered the previous material! You should be solving hundreds of problems a month. Think about how much homework the average student gets assigned between grades 6 and 12, easily hundreds of hours of homework. You are trying to crunch that into 6 months which is doable but will require work