I'm not exactly sure what county Loch Haven is in - in PA, but if he died (your gg grandfather) after I believe 1903, you could get a copy of his death certificate and on it might be the name of his parents.It's worth a try! If it's before that date, you'd have to go to a Library there (do you live in PA?) and check out the newspapers on microfilm.Possibility it might be there.Also check the census.
Don't exactly know the reason for the different spellings of Dershem.There are a lot more than those 2.One reason could be that when the Census takers (back in the 1800's) wrote down the name, they spelled it as they heard it.Many people couldn't read and write, so they wouldn't know if the spellig was correct or not.
No, never heard of the story of 2 German brothers coming to America.Could be a different branch altogether.There are Dersham/Dershem's all over the US.A lot in Ohio too.