One of the reasons for a traffic drop can also be due to your site losing links. You may be seeing a direct loss of that referral traffic, but there could also be indirect effects. When your site loses inbound links, it tells Google that your site isn't as authoritative anymore, which leads to lower search rankings that in turn lead to traffic drops (because fewer people are finding your site if it's not ranked as highly and more).

To do this, I often align the launch of my content with a couple of guest posts on relevant websites to drive a load of relevant traffic to it, as well as some relevant links. This has a knock-on effect toward the organic amplification of the content and means that you at least have something to show for the content (in terms of ROI) if it doesn't do as well as you expect organically.
Though a long break is never suggested, there are times that money can be shifted and put towards other resources for a short time. A good example would be an online retailer. In the couple of weeks leading up to the Christmas holidays, you are unlikely to get more organic placement than you already have. Besides, the window of opportunity for shipping gifts to arrive before Christmas is ending, and you are heading into a slow season.
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.

Otherwise you might end up polluting your website’s traffic. The pages filled with obsolete or low quality content aren’t useful or interesting to the visitor. Thus, they should be pruned for the sake of your website’s health. Low quality pages may affect the performance of the whole site. Even if the website itself plays by the rules, low-quality indexed content may ruin the organic traffic of the whole batch.
Secure (https) to non-secure sites (http): Since Google began emphasizing the importance of having a secure site, more websites are securely hosted, as indicated by the “https” in their URLs. Per the security protocol, however, any traffic going from a secure site to a non-secure site will not pass referral information. For this issue, you can correct by updating your site to be secure through a third-party SSL certificate.