If I'm understanding your landscape correctly you have the project track import into your Dev maintenance track, before going to Prod? Not really a dual track logistics wise, but I may not be 100% clear on your tracks.
Here is how I think of Pre-Prod playing into the transport process:
Dev (main) -> QA (main) ->
Pre-Prod -> Prod
Dev (proj) -> QA (proj) ->
In a scenario like this, I would only need to refresh pre-prod when a project from QA was released; or perhaps numerous snote updates on the main track. When all of your objects have the same versioning, you wouldn't need to refresh as your main track is in line with production.
I also don't think there is any one right answer to your scenario, when it comes down to is either technology limitations or political limitations...more towards the latter.