If you're an emacs user, prefix the line with the desired number of spaces then type M-q, copy back into your browser. That's my solution to formatting block quotes at least. Your long line prexixed with 3 spaces in emacs:

    So a paragraph that would run off the page and break mobile
    devices in normal circumstances should be broken in several
    places by a hard 'return' plus more spaces.

And your split-by-hand block quote:

    Is really just a collection of sentence fragments that all
    fit the same formatting. There might be a better way, but I
    don't know it!