The answer to 2) surprises me. I mean, they only said MIT and didn't explicitly state that they provide the source and didn't say that they use a library under GPL. I guess if they provide the source with the binary and make the licenses of the used libraries clear it is ok? Having your part under MIT doesn't change the license of the library. MIT is not more but less restrictive than GPL.
Also shouldn't the answer to 5) be: Depends on what exactly the licenses of the used libraries are, but usually lol no?
> and didn't say that they use a library under GPL.
The situation is exactly that: a GPL library incorporated into an MIT project:
"You wrote a library in Rust and uploaded it to Cargo using the GPL license. Someone grabs it via cargo and uses it in their own project, which is distributed with the MIT license."
I see. Thanks!