You just answered your own questions.:) Yes to all of your explanations. Example: 她是林老師的太太。She is Mr. Lin's wife. 林老師的妻子在銀行上班。Mr. Lin's wife works for bank. 她是我的老婆。She is my wife. 林將軍的夫人唱歌很好聽。General Lin's wife sings very well. However, I'm not too sure of the differences on word usage between Mainland and Taiwan, might need more answers from users on italki.

For 妻子 and 老婆, you are right. 太太 is formal and polite. 夫人 is too formal for daily life. You could say 这是我的太太。 But it's wired to say 这是我的夫人。It is a bit out of day, and it could make people think you are well educated and you speak 文绉绉地（I don't know the term in English）。

老婆 媳妇(儿) 妻子 太太 夫人 are the same or even you'll see 内人 or 娘子.they're rare now. in oral chinese,老婆 媳妇(xifu)/媳妇儿 妻子 are common. in formal situation,we often use 夫人 and 太太 such as 米歇尔是奥巴马的夫人 Michele is the wife of Obama. 是的,侯爵夫人(answer her requires) at your service,marchioness. BUT you sholoud pay attention that 夫人 and 太太 can conjugate with surname but 老婆 媳妇(儿) 妻子 can't. ex:you can say 马夫人or 马太太 it means the lady whoes husband is Mr.马 BUT you can't say 马老婆 or 马媳妇 or 马妻子 (p.s.:马家(的)媳妇 is correct but it's used only in oral english in chinese counties) i assume you'll see this use in some chinese films. minho's answer is good.

中國的親屬稱謂 是講求長幼尊卑倫理關係的 因此親屬稱謂之使用要掌握 自稱 (老婆)或他稱(夫人) 謙稱(內人)或尊稱(嫂夫人) 的原則

"the most common" depends on the situation you have all the necessary words to most daily situations