I really don't think it would be wise to take an actual test before even taking one PTs...

If I were you, I would finish the core curriculum, take at least a few PTs and see if I am scoring consistently within the score range that I want to be in. Even if you're applying this cycle, August test can still be used for this cycle.

3 billion years ago, it would have been necessary to have higher green house gases than we do today. Only two examples of greenhouse gases that were provided in the passage are methane and CO2.

ex) today's level of green house -> 100

3 billions years ago, which should have been much higher -> 120

argument conclusion: because we see that 3 billion years ago the number is higher, it is safe to say that CO2 level is higher.

But what about methane? what if methane accounted for more back in the day?

B) says methane level was much higher 3 billion years ago. Assuming that is true, we can now say that it is not necessary to have higher CO2 level. Maybe higher methane accounted for all the increase in greenhouse effect.

I think when the question writers said "representative of the population of the whole," it implies the assumption that it is a fair representation of the population.

My advice would be: don’t try too hard. Don’t think about it too much. Your anxiety is a sign that you care! And by showing that you care, you have proven to yourself that you would commit to the world of LSAT. And your achievement is a testament to that. Even if you want to, your effort so far wouldn’t just disappear. Even if you wanted to, skills you honed and refined wouldn’t betray you in the day of the exam. Take a deep breath and take a walk! Maybe get a frozen yogurt.