Why is it not "Tutatkuja Dodomani"? Do proper place names not get "-ni", or do I misunderstand the use of "-ni"?
You are right - you can't add "-ni" to proper nouns, including countries and cities.
How would I say, "we will come from Dodoma"?
I think you would say, "Tutatoka Dodoma". (like "unatoka wapi?" = Where do you come from?)