and its easy to see how thorp, when spelled with the throrn--Þorp became Dorp in africaners
No, thorp is common Germanic, meaning 'village': it existed in Old English as that and also metathesized to throp (in place names such as Winthrop, Adlestrop, and the Spencer pronunciation of Althorpe as Althrop). It also occurred in Old Norse, so Danish settlements in England used it.
th changed to d in the common ancestor of German and Dutch (and Afrikaans), after it had parted company with Old English: thus dorp. It's a sound change, nothing to do with the letter thorn. As you travel south from Holland across Germany you get further sound changes gradually coming in (the "Rhenish fan"). There's a line across Germany south of which the word is Dorf. These are of course common in Dutch and German place names.
