A good angle for ramps is 40º or maybe a little less. They should be able to make it up even if it's dry. As far as the housefly larvae are concerned they can climb straight up the sides if there is condensation present, same as the BSF. Also, the BSF won't eat the living housefly larvae, but they would eat them if they died. I think it's more likely that the housefly larvae would pupate in the unit and emerge later. Either way it should only be an issue before you establish a dense BSF colony.
Your method of using a water mote with the plywood over it is a good one.
Yes, small grubs = small adults. In their adult stage they don't eat so they must live off their stored fat. I believe they need to drink in the adult stage. I think I saw one lick some strawberry jelly once.
