Apologies if this appears twice - I thought I had replied a few minutes ago. In Plakias right now and just a few minutes ago returned from a drive to Sfakia.
No road to Loutro. You can walk after parking up at various locations near Sfakia. Best way is via boat from Sfakia - very short trip. I think there is or was a weekly trip to Gavdos from Plakias - vaguely remember seeing a poster. Might be too late in season now.