Thanks for the write-up. While it's a spooky story from the perspective of his loved ones, I must say that I don't see a lot of reasons to suspect foul-play though.

Ultimately every scenario which involves a perpetrator and Mateusz as the victim of a crime seems to raise more questions than it provides answers. It has very little explanatory power as far as the secured facts are concerned, it doesn't answer potentially open questions but introduces new ones. That he had no obvious reason to commit suicide says very little, as this is what family members and friends think in many cases. The only person I know who committed suicide was the notoriously funny class clown without any apparent financial or family problems. That his car wasn't found doesn't actually seem all that mysterious either, if he planned to end his life he may not even have locked it, and no offense but it's Poland. Or maybe he tried another suicide method first, related to the car (drove it into a lake etc). And if he ultimately planned on committing suicide, it isn't all that surprising that he may have decided to do it where he grew up - a sort of sentimental decision.

Likewise, all open question get even more difficult to answer if other people are involved. If it's already mysterious how Mateusz was able to enter the barn without anybody seeing him (I don't think it is all that mysterious), then it would be even more unexplainable how he and a further person(s) managed to do it. The tickets would merely require him to 'lie' about his whereabouts, which is a very straightforward explanation. How does a second person/perpetrator fit into this? Did they force him to stay in Germany and lie to his family? Probably not. There might be minor open questions about potential additional injuries. But it doesn't really seem like a homicide theory wins us a lot in terms of explaining more things with fewer assumptions. It would answer a minor question with a lot of speculation (teeth out due to attack?) but offer more complicated and speculative answers to questions which seem to have straight-forward answers right now (stage hanging? bring Mateusz to his family's home and in the barn? motive? who? hold him in Germany, make him lie about whereabouts?....).