I'm not familiar with "prescriptive easements", but have dealt with "ingress/egress" easements here .... and CA might be different. The register of deeds in that county is the first place I would check to see if an easement has in fact been legally registered and I would definitely consult an attorney that knows the issues there. Good luck!
Who owns property B and what is shown on the deeds for A and B?
One other thing .... properties can't be "landlocked" but just because C has a house in the way doesn't make it such imo.... that's their problem and likely a driveway "could" fit between the C property line and the house too.
Neighbors suck
Check the R of Deeds web site... most info is online too.