Hi,
Your render time in progessive is set to 1 minute, so small areas will render cleaner then bigger ones. Up that number, or change to bucket mode.
Good luck
Easy solution would be to convert all your bitmap paths (also IES/HDRI/Proxy/Xref) to UNC (network path)
You could also copy all your files from your workstation to the exact same location on your slave (requires sync if you change something)
NAS/server is also an option but might require a gigabit network and for example SSD's if you have those in your workstation and are afraid of speed loss. Also in this case you need your paths to be UNC.
Try UNC first, if that doesn't work there could be permission problems.
Good luck.