Visum für meine brasilianische Freunden verlängern?

The visa cannot be extended if the following reasons are not present:

In what cases can a Schengen visa C be extended?

These special exceptions include humanitarian or serious personal reasons as well as force majeure that prevents your departure in time.

• Higher violence: examples of force majeure are strikes or storms that expose the air traffic, so that you cannot enter your departure. Or a pandemic, like the Corona crisis.
• Humanitarian reasons: this includes, for example, a disease that makes your exit unacceptable.
• Main personal reasons: These may be serious events that were not foreseeable before your entry and which make your departure from the Schengen area impossible.
• Late arrival: Another reason for a Schengen visa extension for the C visa may be a late arrival, which is why you cannot fully use your 90-day stay. In this case too, your Schengen visa could be extended so that you can stay in the Schengen area for a total of 90 days.

Source

Now we wanted to get married in Denmark, as it seemed easier to do it there. Unfortunately, we have been waiting for the commitment for more than a month and it can only remain in the Schengen area until mid-September.

In theory, you can of course marry with a tourist visa or visitor visa in Germany, but then you will have difficulty obtaining a residence permit issued by the aliens’ authority for the repatriation of spouses. As a rule, the aliens’ authority will insist that the visa procedure is enforced. So it is best to make an application for a marriage visa from the start.Source

Conclusion: Your project will face great difficulties. If the correct path with the marriage visa goes, it gets a residence permit and can look for a job and does not have to work illegally. You don’t really want to.