I too am searching for Shippien information, however a different person.
My Shippien woman had children out of wedlock as well.
I have seen the birth record of your grandfather and your right, no father is listed - mother is listed as unmarried, so it was common to take the mothers surname, in this case Shippien.
If the father is not listed in the records, its very unlikely you will find anything about him. It was common for a child to take his/her step fathers surname, in your case Umlah - which would explain why when he went to get his pension it came up "shippien". Because of this it sppears that George Umlah did not legally adopt him, but rather "gave" him, his surname.
As I have noticed Shippien is not a very common surname. We very well could have a link to each other somewhere down the road.
Feel free to contact me if you need any help :)