even seomoz.org is shown when you search it with site-parameter.
No error as long as it is not shown for search queries

target for optimization may be: Make it easy for a user to login with all devices

Thats a really good question and I can only tell you my own point of view. Even in SEO-world in done and not done. I just think like this:

According to Google, use nofollow for

• not trustworthy pages you link to
• paid links
• crawl prioritization

So one and two  is not the case (hopefully I can trust your socials). 
To Point three, some webmasters really want to safe "PR" or "DA" with not passing it to pages using nofollow. This is not working, the PR is just send to nirvana. You can't save it, it is divided by links - no matter if the are followed or not. So the only reason in point 3 is - you don't want Search Engines to notfollow and notindex your socials (last is done if you nofollow them or not) - so my Point of View:

I just use nofollow in cases Google says I should (you can btw read it here https://support.google.com/webmasters/answer/96569?hl=en). Socials is not one of these cases, so I use Follow .

Add: Comments and User-Generated-Content is also a good Idea of using nofollow - dont know if Google mentions that on the given Page)

Dont just copy and paste withoutout canonical - thats duplicate content.
I would translate as much as possible just to serve my usuall quality to spain. Why should I do less than I can? I like to do things right - not just do them...

Don't forget international SEO (hreflang a.s.o): https://moz.com/learn/seo/international-seo

Thats what I would do. Maybe I wouldn't have used a new domain - like to use one .. thats me, what are the others thinking?

Google Analytics will track everything if it is implemented and it is working. It doesn't matter if the page is noindex or nofollow. That has nothing to do with Google Analytics. Not sure what you mean with "external site links" - but whatever you mean, guess you do it on your follow/index pages, so it will work.

Links from xyz.wordpress.com and abc.wordpress.com are 2 different links from 2 domains. Not one of them has the "DA" of www.wordpress.com because thats also a different domain.

We can say, that google is seeing it that way and not. Not because, in search console linking domain would be "wordpress.com". Noretheless they are different in a lot of points, DA is the most important one.

If you get more links from the same domain, it makes them less and less important (for every new link, not the old links). In my tests (wordpress & blogspot) it was the case, that it was less and less for the one linking subdomain, not all the sub-domains. Somehow understandable? So the answer is "it depends" The test is not 100 % accurate and hard to compare but seems like thats the way it is.

What I also realized, when I get high increase in organic traffic and I have done nothing and no update was going on, it is mostly one new organic link. You can build links as much as you want but organic links beat everything and I dont know how google can figure that out so clearly.

So when you have a good amount of links, you should focus on your user. Like I do, thatswhy I cant tell you how MOZ or any other Tool is handling it, I simply do not care that much.

One of my former competitors has 10 times more Links as we have and today I call him "former" competitor. Former because we reached a new level, he not. We have now 9times more organic traffic and he still stuck where he is since a year. Well we have a single Page with more monthly organic visitors than his domain. We started on the same level 1.5 years ago. I did not build one single link, I just focus on users. A lot users, talking about hundrethousands organic visitors, for germany in this niche a lot. Ok we are now going into other topics, but thats not the point here.

All these topics are finance topics so maybe a YMYL special thing, but I have some more domains wich work better and better without building links and non-YMYL topics. But there is a lot great content wich makes the domains earn links. At least you need an entry, a dooropener to get links coming in.

Woop - to much hah So you are somehow right, but dont think to much link Anyway - Good Luck!

I bet you will find noone who tested it. I can just tell you, what I would do and why.

I would take a new domain and here is why: Using a subdomain is like using a new domain. so why take a risk when not needed? I mean, in theory, it is like a new domain. It should be possible to pick a subdomain without problems. But I wont do that because I don't know how Google is treading porn-sites really.