You can edit your files locally in a text editor, make sure to save them with a utf-8 encoding and then upload them to the PCIbex Farm, or replace your characters with HTML entities like: óñ etc.
I have a similar problem with French characters. I have tried saving my CSV with a UTF-8 encoding and reuploading and have also tried replacing the characters with HTML entities from the list you provided, but neither seems to be working.
At least one of those solutions should have worked. Maybe the changes somehow weren’t taken into account when you tried. Did you try taking your experiment with another browser and/or in incognito (private) mode?