Love the five different area of investigation that you went over, great way to analyze and diagnosis the issue. I would also definitely agree doing a rankings comparison between the two time frames, and not only check what your Google ranking is, but also track the search volume for your keywords to see if it has fluctuated or gone down. Google Trends is a great tool for this as well, as one of your keywords that your ranking for may have just lost popularity online.
This is the number of views that you can test each month on your website.It's up to you how you choose to use them, either by allocating all the views to one test or to multiple test, either on one page or on multiple pages. If you have selected the 10.000 tested views plan and you run an experiment on your category page which is viewed 7000 times per month, then at the end of the month 7000 is what you'll be counted as tested views quota.