The best versions to stick with are arnon ni and arnyn nhw - these are the colloquial usages taught in Welsh for Adults and in schools, and will avoid confusion with any formal or dialect forms. If Duo offers up anything else, just raise a query through the report function.
There are other, formal forms in some dictionaries etc (arnom and arnynt in this case), but you are unlikely to meet them in this course or in the wild until you start reading more formal texts.
Thanks ibisc. When I wrote the comment, I'm sure I double and triple checked the table, which I'm sure had arnon for both ni and nhw, but having just gone back to it now, it does have arnyn for nhw, so either it's been changed or I'm going mad, or both! Thanks for your help.