listen, friend, i think you found bad+scary info on the internet, and its causing you to post a lot of wrong info. if you just try to flash it, you will almost certainly succeed, and if you fail, THEN the forum can help you.
also, T-Mobile in USA works pretty well on the 10III, and nothing else with SFOS support does. MMS even works pretty good these days, although the mobile coverage isnt as good as north american android phones with LTE band 2