Is it permissible to pray Witr three rak'ahs with two salams, or with one salam if one is in a hurry and fearing her husband? And is it obligatory to repeat the prayer if, after finishing it, it becomes apparent that the recitation concluded with verses from another surah?

Both forms of Witr prayer are valid. The first form is to pray two rak'ahs with a salām, and then one rak'ah. The evidence for this is what Al-Bukhari narrated from Abdullah ibn Umar. As for the second form, it is to connect the three rak'ahs with a single taslim. Ibn Al-Qayyim mentioned in Zad Al-Ma'ad that the Prophet, peace and blessings be upon him, used to pray Witr with three rak'ahs without separating them. So, both separating and connecting the rak'ahs in Witr are valid. As for moving from one surah to another during prayer, there is nothing wrong with it, especially if it is unintentional, because reciting a surah is not obligatory.