> No no, no such thing as "good faith" is applicable here. Good faith
> does absolutely not make it *legal* to use other people's copyrighted
> materials, not in any way.

As you're not exactly known as legal expert, please have a read:

https://en.wikipedia.org/wiki/Good_faith_(law)

Both Stuntz and Maus/Böckelmann are German, so this applies here:

https://de.wikipedia.org/wiki/Treu_und_Glauben

> If laire, piru, kiero or jacadcaps (and probably others) should decide
> to make an official copyright infringement thing about this, they would
> probably have a pretty solid case

Yes, against Stuntz that would be, in case Maus/Böckelmann can prove he licensed their code to them. Of course, as soon as it was proven that Stuntz illegally licensed other people's IP without their consent, Maus/Böckelmann would have to stop distribution of their MUI version.

> using their work without their explicit permission is simply illegal.

...as is licensing their work to others without their explicit permission.

> How sources were *obtained* is perhaps of lesser importance, unlike the usage of them.

"Perhaps" is the key word here.

> In the source tree they also found the MUI4 sources. That is also obvious.

No, that's just one version of the story. The accused tell a different story.

> Under any circumstances it falls onto *their* responsibility to ensure 100% IP
> integrity in what they release, when using external codebase.

Please do as suggested and read about "good faith" or better "Treu und Glauben".

> Had they had a valid *license* to MUI4+ [...], they wouldn't have had
> to resort to "filling the gaps" with their own made up code, they could
> simply have based their port on the official sources they should have
> approved access to.

As you know, the accused claim they got from Stuntz a license to 2009 MUI4 source code, so it's obvious they had to "fill the gaps" with their "own made up code" in their attempt to match anything that was added after that to the MorphOS MUI4/5 source code.

> The same with MUI5, had they really had a valid *license* to MUI5,
> there would have been a proper port from the official sources.

As you know, the accused never claimed they had a license to MorphOS MUI5 (i.e. 2015+) source code.

> They created a new branch of MUI development in 2015

2013.