Looking more closely at the patch:

* for the missing tracebacks, you're probably hitting the note in https://docs.python.org/3/c-api/exceptions.html#c.PyErr_NormalizeException and need an explicit call to PyErr_SetTraceback (My recollection is that I made this same mistake when working on the codec chaining patch, so we may want to revisit the comment before PyErr_NormalizeException that suggests making setting the traceback attribute on the normalized value implicit)

* to trigger the implicit chaining in PyErr_String, try calling PyErr_Restore on the normalized exception before calling PyErr_SetString, rather than just dropping the references. That should let you drop the subsequent explicit PyErr_SetContext call.