Sorry for the delay in contributing...just recovering from having a metal pin taken out of the tibia at Portimão hospital on Thursday! ;))))
I have used EVA/DAMAS from Lagos to Seville many times!
There are NO onboard toilets!!!!
As regards drivers, on the EVA buses from Lagos to Seville to Lagos, there is usually a driver change at Albufeira or Faro Bus Station. One can usually request the driver to let one have a brief toilet stop at Faro or Tavira. The EVA buses also stop for up to 20 minutes at the Cartaya Service Area about 20 kms before Huelva.
On the DAMAS buses, one driver usually takes the bus from Estacion Autobus Plaza das Armas in Seville to the Bus station in Huelva and from Lagos to Huelva on the return where there is a driver change-over. There is usually a 20 minute layover in Huelva. Again one can usually ask the driver to wait for a quick toilet stop at Tavira, Faro and Albufeira Bus Stations..