Yep, that sounds about right. The threads on the hub nut are most definitely 'one use' only, as soon as they're torqued up they stretch, and shouldn't be used again. That 'loosening' is it skipping a thread, so you'll never get it torqued up properly.
SEAT should have plenty of these in stock though, they have to be replaced for all sorts of jobs, including the ball joint as you say. Come to think of it, GSF and Eurocarparts should also have them.