I have posted photos of the bus line schedule boards in the past. A big problem especially for the second class bus is that the times on the board are often off by as much as 1/2 hr. or sometimes cancelled without any additioal notification. AUTOCAMIONES DEL PACIFICO Is the company, call them for info. AND, NO, they do not have bathrooms on all of their busses. There is a bus stop in Tomatlan about 2- 1/2 hrs north. The stop is about 10 minutes and there are very clean facilities there.
The bus leaving Melaque passes La Manzanilla about 25 minutes later. Also there is one direct express bus in the afternoon, ya might ask about that when you call.