Historical differences in usage of "Mrs" for "mistress" or "missus"

According to the following source the change from Mistress to Missus took place in the 18th century when the two terms were gradually used with two different meanings, apparently for no clear reason:

Missus:

• corruption of mistress; as oral form of Mrs., from 1790; the missus "the wife" attested by 1833.

Mrs:

• 1580s, abbreviation of mistress (q.v.), originally in all uses of that word. The plural Mmes. is an abbreviation of French mesdames, plural of madame, used in English to serve as the plural of Mrs., which is lacking. Pronunciation "missis" was considered vulgar at least into 18c. (cf missus). The Mrs. "one's wife" is from 1920.

(Etymonline)

Mistress vs missus:

• It may come as a surprise that ‘Mr’ and ‘Mrs’ aren’t actually short for mister and missus – they were originally abbreviations of master and mistress. ‘Mistress’ used to be the title prefixed to the name of a married woman, a usage dating back to the 15th century, as these early examples in the Oxford English Dictionary (OED) show:

  • 1463 in S. Tymms Wills & Inventories Bury St. Edmunds (1850) 36, I … be qwethe to my maistresse Clopton a spoon of berell.

  • 1471 J. Paston in Paston Lett. & Papers (2004) I. 443 Iff it come to Mestresse Elysabeȝ Hyggens … sche schall comveye it to me.

• The use of ‘Mistress’ to denote a married woman is now the least common meaning of the word: it only survives as a dialect form in some parts of the world.

When and how did Mistress divorce Mrs?

• So, the abbreviation for ‘Mrs’ survived, but not the practice of pronouncing it as ‘mistress’: today, the only standard, accepted pronunciation is ‘missus’. But how and when did this happen?

• According to the OED, around the 18th century, ‘missus’ first became an informal contracted pronunciation of Mrs., and ultimately, the only allowable pronunciation. When this stage was reached, Mrs (pronounced ‘missus’) became a distinct word from ‘mistress’ The evidence of when these changes happened is lacking, but J. Walker, in his Critical Pronouncing Dictionary of 1791, notes that mistress as a title of civility is pronounced missis, and that “to pronounce the word as it is written would, in these cases, appear quaint and pedantick”…’

• So, we’re not sure when Mrs and Mistress became two separate words, pronounced differently – but we know that this has been the case for at least 200 years.

(Oxford Dictionary, Language Matters)

The OED specifies 16 principal meanings of the word mistress, and in addition many sub-meanings. The two to which you refer are included in that number.

It is sense 2a which refers to a wife.

a. The female head of a family, household, or other establishment; a woman holding such a position in conjunction with a male counterpart.

The examples given are as follows. You will notice that the those containing the term mistress as opposed to Mrs are very sparse since the 19th century.

a1375 William of Palerne (1867) 1016 (MED), Alisaundrine..attlede þe soþe, þat hire maistres & þat man no schuld hire nouȝt misse, þeȝh sche walked..from here siȝt.

a1425 Rule St. Benet (Lansd.) (1902) 10 (MED), Ilkain sal take discipline at oþir, als hir mastiresse þoȝ scho ware.

1483 (▸1413) Pilgrimage of Soul (Caxton) iv. xxxviii. 64 She bare hyr seluen boldely, right as she were maystresse, and hadde alle the gouernement of the kyng, and his houshold.

a1513 H. Bradshaw Lyfe St. Werburge (1521) i.xxi. sig. g.viiiv, Of whiche sayd places [sc. monasteries], she had the gouernaunce As worthy maystres.

1584–5 in J. D. Marwick Extracts Rec. Burgh Edinb. (1882) IV. 400
The maister or maistres of euery house.

1611 Bible (King James) 1 Kings xvii. 17 The sonne of the woman, the mistresse of the house, fell sicke.

1641 J. Jackson True Evangelical Temper iii. 225 The Mistris is a good Huswife, but of shrewish condition.

1711 R. Steele Spectator No. 202. ⁋ 12 That the Masters and Mistresses of such Houses live in continual Suspicion of their ingenuous and true Servants.

1773 H. Chapone Lett. Improvem. Mind II. 72 The mistress of the family must be ever watchful.

1814 Scott Waverley III. vi. 80 The future mistress of my family, and the mother of my children.

1861 F. Nightingale Notes on Nursing (new ed.) ii. 24 The mistress of any building, large or small, does not think it necessary to visit every hole and corner of it every day.

1896 J. M. Barrie Margaret Ogilvy ii. 27 She was eight when her mother's death made her mistress of the house and mother to her little brother.

1921 A. Huxley Crome Yellow 131 The young lady accepted him, and in less than a year had become the absolute mistress of Crome and her husband.

1997 P. Carey Jack Maggs xlii. 151 It was as clear as day to her that she..might one day be mistress of the house wherein she had been called to serve.

The other meaning to which you have referred is number 7.

> 7. A woman other than his wife with whom a man has a long-lasting sexual relationship. In early use: †a woman notorious for some act (obs.).>

There are entries for this from as early as the 15th century.