Fascinating story! Wish it were my Prossers!
My GreatGrandfather Joseph Prosser also came from Wales; guesses are Monmouthshire or vicinity... I haven't been able to find when he came to the US (although he was in Pennsylvania in 1860) so I can't make the connection to his family of origin.
Joseph Prosser was born about 1829, emigrated to Pennsylvania, evidently living first in the Minersville area, moving to Scranton, then to Ohio, where both he and his wife Mary Ann (Morgan) Prosser died. They left at least 3 children, William Byron (my grandfather), Joseph Burns Prosser, and a latecomer, Cordelia. The three children were sent to the Soldiers' and Sailors' Orphanage in Philadelphia. There may have been another daughter, or Cordelia may have also been listed as Mary at one time.