I am not 100% sure if this is the cause but when I look at the source code I see the google analytics tracking code on the page. The other reason could be you have a custom report set up to collect data about 404 pages but it does not appear to be the case when looking at the attached image. I would recommend putting a 301 in place for that page since it appears to get a lot of traffic.

Hope that helps some.

I think the easiest way to figure out whether or not Page Rank is properly flowing to those deeper sub category landing pages is to crawl the site with Screaming Frog.

I don't remember if a Moz crawl pulls page rank for every page but if not crawl your client site with Screaming Frog and connect to Moz's API. This will pull in the Page Rank for every page and you can sort to find out if there are any issues.

How does the hamburger menu look like on mobile? I'd be curious to see what the engagement looks like when the entire menu opens by default.

I would leave all the disallows out except for the /wp-admin/ section. For example, I'd rewrite the robots.txt file to read:

User-agent: *
Disallow: /wp-admin/

Also, you kind of want Google to index your cached content. In the event your servers go down it will still be able to make your content available.

I hope that helps. Let me know how that works out for you!