There will be second-class departures not shown on the ADO online schedule, and colectivo vans. In Escarcega there are two bus stations: a central public station that serves several carriers; and an ADO station at the west end of town, a couple of km away. You will have to decide whether to wait at the desviacion for Calakmul for a passing bus or van, or return to Xpujil and pick up a vehicle at the start of their trip.
It´s about two hrs from Escarcega to Campeche, and an hr 15 min from the desviacion to Escarcega. There are one or two decent hotels in Escarcega centro, and some more at the west end of town. But as long as you´ve picked up a ride to Campeche before dark, you should be fine.