I'm finding only 1 Per/Parry JOHNSON in Mahaska Co IA
1885 census wife is Hannah; 1915 census wife is Johanna SWANSON; 1925 he is 90 and widowed....this census names parents: John SWANSON/Stina? ANDERSON
and he is a Methodist, not Lutheran
Perry says he wasn't naturalized, but there is Nat. Index, Mahaska Co 10-22-1886 for him.
It is doubtful this is the person you're looking for.
I also couldn' t find anything further for Per, Christina, Addi.
There is a Per in La Porte, but b 1852 w/Mari Caroline ANDERSON. I also thought maybe Per died and Christina remarried, and there was a Christina JOHNSON m Daniel SWANSON, but she is too young. Both parents may have died and Adde Albert is living with another family with a different surname.
Any possibility they returned to Sweden, or to your neck of the woods?