You're leaking keditbookmarks

Have you checked errorString actually returns something?

Given startDetached is static it seems kind of weird it would update the "this" errorString

Yeah it returns unknown error, my mistake, I suppose I should just switch back to call without object since it's a static method
There don't seem to be any informative errors apart from FailedToStart (which is the only one we are interested in?) in QProcess anyway

critical is a bit heavy in my opinion, i'd just go for warning for this and the debug line below.

Why const_cast?

Uh oh, I'd assume startKEditBookmarks should be const (which I forgot) since it doesn't change the object, yet we need to emit non-const singal error. So should it be const or not?

Also, I'm looking at slotEditBookmarks and slotEditBookmarksAtAddress, and don't understand why those weren't const, since they don't have side-effects too, although I might be missing something since I'm not too familiar with Qt.

i'd go for non const as the other two methods, yes they could be const since they're not modyfing the object, i guess what the original author had in mind is a meaning of "this changes things eventually" so that's why it isn't const, or maybe it was just a mistake :D