... (In case Tom has moved on. He's as busy as a bee, I'm sure!)
If Tom replies, that's great! Combine my info w/his and go from there.
You don't necessarily have to lay down rosin paper unless you know, or suspect, a moisture issue from below. In short, if the room below is thermostatically controlled like the room that has the flooring, you should be OK to NOT use the paper because the floor should maintain a pretty even temperature. On the other hand, if the room below is an unfinished basement that is 'lightly' heated, consider using the paper because where warm air meets cool air, you get condensation/moisture.
RE: Using I don't see why you can't install the new flooring on top of the existing plywood subfloor. 1) As long as the plywood is 3/4" thick (or as long as it meets the hardwood's MFGR 'thickness' requirement for a subfloor, you can go ahead and do the install over top of the subfloor. 2) Since you don't want the new floor's planks/strips running the SAME direction as the sub-floor AND, with that, the planks/strips will run in a (finished) direction that you desire, then go ahead and install the new floor over top of the subfloor.
As an aside, I installed strips in our foyer. Unfortunately, the way the joists and subfloor were 'laid out', I had to install a 1/4" thick plywood laminate on top of the existing subfloor (which I installed 90 degrees to the existing subfloor) so I could run my strips at 90 degrees to the 1/4" subfloor. See what I mean? In short, you don't want any 2 'layers' running the same direction. They should be 90 degrees of each other for strength.
Consider screwing your subfloor down to the joists before your install. (Pre-drill too.)
Wait to see if Tom has anything to say/add. My best to ya and hope this helps.