Thank you for researching this topic!
I think you didn’t mention the right Luca* at the start of your post 
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For reference, here is the inline link topic opened before.
Some accessibility features are not improvements for everyone. For example, if I remember correctly, we increase the default text size as part of the extra accessibility features. Some users might prefer it with the smaller text.
From my understanding, in those examples ‘consistency in the way links are displayed’ is lost in profit of ‘adaptability’ (similar to the solution proposed on XWIKI-20827: Inline links must be distinguishable without relying on color by Sereza7 · Pull Request #2313 · xwiki/xwiki-platform · GitHub 
). Seems like the solution we converged to is a compromise a lot have already chosen.
So for regular users it would mean having a dropdown with only one item. I think it’s a bit too much. Moreover, as things are now, there isn’t really ‘enough’ in the accessibility features to justify giving it so much importance.
I think this idea is nice, but we’d need more user-level customizations for it to be useful: toggle for a dark mode, high contrast mode, font selection, … Even with all of those, I don’t think it’s important to keep it on the page at all times, so I’d probably put it in the drawer in the section related to the user profile. This way, most users can see it, but it doesn’t clutter the page in most use cases.
Overall, I’m -1 for this, even though this concept is interesting for the future.
Navigation should be consistent, I don’t think having it disappear is a proper solution, even if we notice the user. An extreme case of this would be ‘The breadcrumb on every page disappears on the first of April.’ Users have their habits with the interface and breaking them for arbitrary reasons is not something we should plan on doing.
Yup, menus have accessible submenus since 15.3
ATM there’s no underlines, and for WCAG, we want to underline links that are inline (links that are surrounded by text). This is pretty much just the same as what they do in the screen you shared from Obsidian. See G183: Using a contrast ratio of 3:1 with surrounding text and providing additional visual cues on focus for links or controls where color alone is used to identify them | Techniques for WCAG 2.0 for more details about this criterion.
The two main issues we discussed that made this change difficult were:
- underlining every link is ugly
- if underlining only a few links, we have consistency issues
We agree on the first one, and from what you showed, it seems to me like the second one is an acceptable drawback.
I think we can assume safely that the Admin users are able to navigate their way through the admin panel and search on the documentation.
And there’s a Help section on the default flavour to guide new Admin users through this process.
Thank you again!
Lucas C.