I don't know WA law so I can only give you generalities. Also, if they brought an action you should have a local real estate attorney who should know better than anyone on these boards.
In general terms, first, hostile usually means without permission so if they just did it, even if there was no anger or hostility involved, as long as you did not give explicit permission it was probably hostile. Usually the easement they could gain would only be on the part they actually used but as you point out it would become their word against yours. I assume the "piggyback" you refer to is tacking. My guess would be though that you are correct in that the tacking can not go back past the point where there was a common owner. You couldn't claim hostile use of your property by the person who owned it and that would have to be the claim if someone tried to tack back to a time during which one person owned both lots. How long has it been since the lots were split? You also might want to double check the 7 year requirement. That sounds a little short, might it be 7 years if the use is under color of title? Typically uses under color of title have a shorter time requirement (often 5 to 10 years) while uses not under color of title require a longer time period.
I would talk to a local attorney to have a better idea of the strength of their claims. If it looks like they do have a case they might win you could always negotiate with them to limit the easement or maybe get some payment from them in return. It will cost a lot of money on both sides to litigate and you might be able to come to some kind of agreement and save yourself a lot of money and headaches.