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?

Yes. The fact that the MIT license is compatible means that the MIT license allows you to fulfill the terms of the GPL—you can fulfill both at the same time. However, fulfilling the (more permissive) terms of the MIT license does not mean that you have fulfilled the terms of the GPL.

A GPL-non-compatible example would be a Business Source License or something non-free. It’s impossible for you to merge a BSL project and a GPL project, because the GPL would require that the result be usable for any purpose, and the BSL would require that the result is not used in production.

I would like to know too. I suspect the question should have been phrased "they offer pre build binaries but no source code".

it is because GPL is viral

https://www.gnu.org/licenses/gpl-faq.html#WhatDoesCompatMean