I'd take the parallel lines to indicate a slight but distinct break in the sound, but without delaying the rhythm - or doing so only very slightly. Any short silence would be subtracted entirely or largely from the time allotted to the final note before that parallel lines, and none at all from the succeeding note.
It's not too uncommon - maybe slightly more common on popular music than classical. A comma above the staff is sometimes used, as far as I can tell, to mean much the same thing - and that would be more in classical scores.
As to the 8va signs - I think they should be indicated separately for both staves - that is, assuming all notes written are intended to sound an octave higher. As I understand it, it is not standard to assume that an 8va sign above the upper staff applies also to the lower one - even if it is in treble clef also, and even if beams join notes on both staves together.
I have occasionally seen music (I tend to associate it with French impressionist music) where the 8va sign appears only above the upper staff, but the structure of the music seems to imply that it is intended to apply to the lower staff, too. But I consider this non-standard, and certainly quite ambiguous. (If you have to analyze the structure of a score to see whether *this* or *that* is intended, there is something wrong with the notation somewhere.)
I have occasionally seen passage work alternating between two treble staves and a single 8va sign at the top - but the dotted line which extends from it weaves up and down, going form the upper staff to the lower, and back again, according to the alternation of the notes on the two staves. Again, I think French composers of a certain period tend to do this. This is better, although still looks odd to me. At least it is quite clearly applying to both staves here; but I would still prefer separate 8va signs, each one dedicated to a single staff - as is done in the piece under discussion here - quite correctly, in my opinion.
Regards, Michael.
Topic: Notation question