Most of those links are going to be nofollow links, so when it comes to building Domain Authority, it's generally not going to be recognized as a link that passes DA. Google, on the other hand, may actually count those links--as we've seen nofollow links help rankings.

As you build links, keep in mind that it's good to drive real traffic (which helps rankings) and social media shares (which helps rankings) and build your natural link profile (which helps rankings). You need nofollow links for your link profile to be natural.

Any guest posting that you're doing and  how it affects your site's search engine rankings will depend on your site's link profile. If you're only doing guest posting and that's how you're getting your links, then that may put up a red flag and it might affect rankings. However, if you have thousands of links from other sites then a few guest posts aren't going to hurt, they should help.

Keep in mind, though, that any guest posts should comply with Google's webmaster guidelines, the blogger should disclose that it's a guest post, and typically it should be a nofollow link to your website. If it doesn't comply, it doesn't matter how many posts you have, even a few could get your site penalized.

When you have a page that's still on the site (even though it's removed from navigation), the page is still on the site and indexed--so users will still get to it. It's still probably going to rank for at least something in the search results.

What you need to do is figure out if the page still contains valid content and is useful for visitors. There's obviously a reason that the page was requested to be removed by the content manager, though.

If the page needs to be removed from navigation, and it won't have any internal links, then most likely you probably need to remove the page, as well. What I would do is set up a 301 Permanent Redirect from that URL to another on-topic page on the site. As long as you use a 301 redirect and redirect it to another internal page on the same domain, you won't lose and "SEO value".

Generally speaking, this is normal, and you shouldn't have to worry about it. There's a Google Webmaster thread that pointed to this issue specifically, and Google gave an official response (https://www.seroundtable.com/google-sitemaps-duplicate-16128.html)

"This is apparently normal if you submit Sitemap files with the same URLs listed multiple times. Since submitting the same URLs multiple times doesn't change anything with regards to your site's crawling & indexing, I'd just submit it once if you want the correct counts (alternately, if the count doesn't bother you, you can also just leave it like this."

Any article that has release date prior to 1st-June-2012 should return a custom 410 page with "noindex" metatag, instead of the actual content of the article.

The error returned should be a "410 gone" and not just a 404. That way Google will treat it differently, and may remove it from the index faster than just returning a 404. Also, you can use the Google removal tool, as well. Don't forget the robots.txt file, as well, there may be directories with the content that you need to disallow.

But overall, using a 410 is going to be better and most likely faster.

Right now there is no tool that is going to show you the DA authority over a period of time. However, you can look at the site's rankings over time if you were to use SEMRush.com. You can also download the site's backlinks, and using most link tools they'll show you the history of the links and when that site acquired a particular link.