How did "everloving" become a completely generic intensifier?
I believe it's a euphemism for "motherf***ing", with a similar sound and a meaning close enough to be understood but not actually obscene.
I agree with Tim Lymington's appraisal, it was the first thing that came to mind -"motherf**king", which was softened to "motherloving". That's still offensive by its connotations if not the hard language in it. Therefore "everloving" is a further softening of the phrase.
This is, of course, conjecture, but I'm happily convinced by it :)