I think it'll round off for sure if you use an adjustable. There are
special "line wrenches" for this type application that are similar to open
end wrenches, except that the ends of the open end wrap around a little,
allowing you to grab the corners of the nut as well.
As to replacement, I can't offer much advice. I've never had to do it,
and I don't recall enough about the vehicle's layout to provide advice.