so opera can break a 9 month old install that's just used for basic browsing?
Not necessarily Opera. It can be caused by OS and/or hardware issues.
I can't just click a "create profile" button so I can experiment concurrently and make sure I don't lose anything, right?
Right.
There are lots of nice hoops to jump through?
Sorry, don't know how to do it on Mac OS. on Windows you just need to rename the profile folder.