Agreed, we will continue to debug the debugger to find out why the printf() throws it all out...

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Regarding the root issue with printf() and the Serial Debugger, it seems that there need to be additional linker flags added if printf() is needed in a sketch for the Mega (which can be seen in the stdio.h files' comments for vfprintf()).  I still don't fully understand why this causes the issue with Serial debugger (which only uses standard Serial.print/read/write methods), however with the additional flags it all appears to run without issue (suggesting printf() is the root cause).

If you add a Local board.txt to your project (vMicro > Add Code > Add Local Board.txt) and paste in the contents of the attached file and save it.  Then rebuild and upload the project with the Serial Debugger enabled, and the {nrf=?} breakpoint set, it should now run without crashing.

I have followed your instructions re the printf problem & I'm afraid it still crashes.
I have checked and the code is there.
I have even stopped and restarted Visual Studio and then rechecked your text is present, rebuilt & downloaded, but it still crashes.
At this stage I am happy to remove the printf statement & leave it to you.
Thank you.