Hi,

Just to let you know it's quite easy to find the domain of the site in question from the screenshot you provided

I think as you've created a new sitemap and submitted it to Google and removed all the pages so they return 404s, you should be fine. The page will just drop out of the index in time. You might also want to add the address of your XML sitemap to your robots.txt file as it's not there at present.

I crawled the site using Screaming Frog and couldn't see any dodgy looking pages remaining. You could create removal requests of the pages in Webmaster Tools if you'd like to take an extra step. Do this under Google Index > Remove URLs. To prevent this happening again, make sure you keep your WordPress installation and all plugins up to date.

Hi,

Although it seems slightly strange, I can't see any significant SEO issues which it might cause. Have you asked the client why they want to do this? As I'm sure you know, usually one page should focus on one topic and therefore require one link from a main navigation.

Something worth considering would be the internal anchor text which Google will take in to consideration for the linked-to page. If you have 2 or more links on one page linking to the same page, Google will use the first instance of the link as the anchor text. This is something you may want to have a think about.