How to sync Iphone on laptop with music collection on desktop?
I dont know about mounting the folder but You can add all the music to the laptop and iTunes, Sync the phone, and once it has fully synced, you can then delete the files from their location, do not delete them in or from iTunes, just the iTunes folder.

Now when you sync again, the iPhone will not try to sync songs as they are already synced and you do not have any music files on the laptop.