Hi Thomas,
As you can see in the answers in this thread, DA can fluctuate with updates and the latest update was April 26. Do you track competitors' sites? That might tell you whether they saw similar changes. One inbound link from a low DA site won't do this. Unless there are other things going on that you didn't mention, this isn't worth worrying about.

I did just this type of thing a little over a year ago and organic traffic is up over 300% now. We made the change mainly to improve the structure of the website(s), with more logical organization and better internal linking. We did do the move all at once (thousands of pages) but it took a lot of behind-the scenes planning to be ready for that.

First came the decisions about what sections and categories made sense for our site. (Using the URL structure to guide users around the site makes it easier for them to find what they are looking for and interlinking between related posts as appropriate is also good—and this helps a lot with search engines.)

Then came the organization of posts into their new categories. To make things easier, we kept the individual path names the same (so www.siteA.com/old-category/old-post-string became www.siteC.com/new category/old-post-string) and uploaded them into their new categories when the time came.

We also used this time to do a limited content review (posts with the most traffic) and we updated a lot of these. We made the choice to keep most of our old posts, even though in our market they can get outdated quickly, to conserve any links we may have acquired. (The main site that we were directing to the new site was pretty old and had picked up a lot of links over time.)

We could have done a more complete content review before the changeover, but in part we wanted to see how these posts did under the new structure—we did get renewed life out of some of them, and we further updated and optimized those.

In conjunction with the export of the old sites to the new one, we made sure to 301 redirect all of the old posts to their counterparts on the new site. For the posts we chose not to bring over, we 301 redirected them to a related post in the same category.

We still occasionally come across things that need to be fixed—old posts that need redirecting/updating or 404 errors that need to be tracked down (one big issue we found was a lot of old pages had old links with hard paths to the old website root domains, causing a bunch of nasty internal not found errors—not good!) but overall we are happy with the change. (Up 308%!)

I don't think you will lose traffic if you noindex the login page. People who are doing a search and then clicking on a login link are very likely to be specifically looking for you and if the login result is not there, they'll choose the next best page, like the homepage.

I second Gaston's comment about intent and usability. If searchers are going to be driven to a different page, you need to be sure that they can easily complete their task there.

I just popped that into ScreamingFrog and I don't see a noindex on that page, but I do see it on some other pages. (Though that shouldn't stop other pages from being crawled.)

Maybe it was just a glitch that happened to occur at the time of the crawl. You could try doing another crawl and see if you get the same error.

Hmm. How about on the page itself? Is there a noindex?

That just says that you are blocking the Moz crawler. Take a look at your robots.txt file and see if you have any exclusions in there that might cause that page not to be crawled. (Try going to yoursite.com/robots.txt or you can learn more about this topic here.)

It looks like the error you are referring to did not come through in your question. Could you try editing it?

If a page has no links and has not been submitted another way, Google won't see it.

It's been a while since that has been considered as a ranking factor (and it never was a big one).

However—depending on what types of pages these are, I'd leave the bolding (unless it is excessive). It could be useful in terms of user experience—as soon as the person lands on the page, they see what they are looking for.

I use CoSchedule to manage social media (and as a content calendar). Easy to use and nice analytics.

Google has a number of ways to find pages, including links from indexed pages (whether internal to the site or external) as well as sitemaps and Fetch as Google.

Most of the time when you come across a page like that (one that has few if any internal links) it's not an organic-traffic oriented page, it is set up for some other purpose, like a landing page for a marketing effort.

The numbers in the MozBar, PA, DA, links, can be found with Open Site Explorer. Just input the URL you want to examine and hit enter.

Right now, the MozBar only works in Chrome. (Scroll to the bottom of the thread to see the latest update.)

I love this add-on (I especially love it when I do a search and see all the data on each result) but you can get the same information by using the tools inside Moz, you just won't see it immediately as you look at different pages.

I myself prefer Firefox and was disappointed when the MozBar stopped working there. So my compromise is that I still mostly use Firefox but just switch to Chrome if I am doing something where the quick information from the MozBar would be useful. You could do the same with Safari.