In article ,
says...
On the inside wall of the oven, (i.e. in the food area) is a shield
made of sheet metal, about 2 inches by 4 inches. The sparking has
occurred at the top center of that shield. Now there is a small hole
at the sparking site, about 1/8 inch across.

How safe (or unsafe) is this oven? Is it fixable?

"Safe" is a subjective judgement. It is fairly obvious that this condition
needs to be fixed. I would recommend not using the unit until it is.

I am trying to guess what the piece may be. Are you sure it is metal? (The
size and placement lead me to think it might be a difuser; mentioned in my
response post a couple days ago)

If it is metal, are there screws securing it to the other metal? If so, try
removing the piece, cleaning the goop off it and securely replacing it. A
metal part must be securely connected to the cabinet wall. Of course the
new hole in it suggests you should replace the part anyway.

These are only long-distance guesses. Good luck.