If I'm picturing this correctly the cars coming down off of the hill have the merge. *Technically* they need to be sure to merge properly with you- the traffic with right of way *BUT* dead and right is not the way to be....

It sounds like a dangerous area that you will always have to be very careful in. Traffic laws totally depend on where you live, but around here bicycles are vehicles - we have all the same rights as cars, so in that situation the car would still be obligated to merge properly with the bicycle - that said way too many people drive their cars quite numbly and they will likely not see you, so be very careful in that situation. Personally I'd probably take the center of the straight lane, watch the merge lane very carefully, signal and move to the right as soon as it was safe. I'd also be diligent about makings sure merging cars could see me - make eye contact if possible, etc.