That's something (interesting) to unpack but wouldn't belong here. Anyhow for the sake of my personal benefit I proceed to comment.
You answered everyting per se. So thanks.
There is a connection between a lot different Software Platforms on the Fediverse, but not every feature is on every plattform. So you are not able to downvote a Lemmy post from Mastodon.
Makes sense: Mastodon does not have downvotes, apparently. But:
You have the option to interact with Lemmy posts with a Mastodon account ...
Is this also valid the other way around?
Are there peer-reviewed documents I can inspect to learn how the fediverse can interact between eachanother? I did not look into the protocol since standards wouldn't address my higher level view I have applied here, I suppose.
I have never used the Mastodon app, i think a lot 3rd Party apps are better on Mastodon, so i don't know how it looks, but Mastodon has, or had, a weird form in showing commet trees in my opinion.
I downloaded the fediverselab app prior but it didn't allow sorting either. Which is why I made the above assumption.
The mastodon app appears to be a very good piece of software; Better then the former.
The app doesn't allow opening links from any instance but it appears to me that they do not want to provide a wildcard intent (android-API specifica) so that they will be mentioned when tapping any link on ones android device. A thing which has to be implemented at build time and would possibly still leak private instance domains.
It looks carefully engineered.
// edit #2: fat font