How was ownership of property managed during the Black Death, when so many original owners had died?

Firstly, it is worth noting that the Black Death actually reached England in June 1348, not in the twelfth century as you stated.

But to answer your specific question, no the idea of appointing escheators wasn't caused by the Black Death.

The system for of appointing escheators was initiated in 1232, and by 1341 had already achieved the form that was to become the regular practice for the rest of the medieval period.

Early history of the Escheators

The office of the escheator was established in 1232. It's role was to enforce the Crown's proprietorial rights in the properties of its tenants-in-chief

In 1232 two escheators were appointed to control escheated revenue. One was given control of the counties south of the Trent and the other control of counties north of it. Under them in each county were the sheriffs and deputy escheators.

• UK National Archives, catalogue description, Exchequer: Pipe Office: Escheators' Accounts Rolls

There was a short-lived reorganisation of the system in 1275:

In 1275 this system of two escheators was replaced by one in which 19 sheriffs performed the functions of the escheators in 29 counties. By 1283, however, de facto there had been a return to the policy of two escheators (but with a separate escheator for London) ...

• Bruce M. Campbell, Ken Bartley: England on the Eve of the Black Death: An Atlas of Lay Lordship, Land and Wealth, 1300-49, p18

The next re-organisation occurred in 1323, when the Ordinance of Cowick led to a major reorganisation of the Exchequer. The original two escheators were reorganised into nine local escheatries:

The Cowick Ordinance of 1323 ordered that because of the increasing volume of information being enrolled on the Pipe Rolls, 'foreign' accounts, including those of the escheators, should be enrolled separately. The earliest individually enrolled escheators' accounts roll dates from then.

• UK National Archives, catalogue description, Exchequer: Pipe Office: Escheators' Accounts Rolls

Under Edward III the system reverted back to the original two great escheators in 1327. This was modified from 1335 to encompass

"... separate escheators for Holderness and a group of four south-western counties comprising Cornwall, Devon, Dorset, and Somerset."

• Campbell & Bartley, p18

In November 1341 the escheatries were again regrouped:

"... to coincide with the shrievalties, an arrangement that henceforth became the regular practice"

• Ibid

Note that a shrievalty here is the jurisdiction of a sheriff.

Inquisitions post mortem

Inquisitions post mortem were:

"... local inquiries into valuable properties, in order to discover what income and rights were due to the crown and who the heir should be."

The inquiries were generally established by order of the Chancery.

"When the chancery heard of the death of a tenant in chief, a writ of diem clausit extremum (‘he closed his last day’) was drawn up, ordering the escheator (the royal official who held the inquest locally) to hold an inquisition in his county."

If the deceased held property in more than one jurisdiction, writs might have been issued to multiple escheators.

However, an escheator could hold an Inquisition post mortem under their own authority.

There may be multiple copies of records of the Inquisitions post mortem. This has helped ensure their survival, and is one of the things that makes them such an important primary source.

• If the inquisition had been ordered by the Chancery, then escheators usually returned all the documents to them, with duplicates (minus copies of the jury lists) being sent to the Exchequer. Chancery records are held by the UK National Archives in series C132 - C142.

• If the inquisition had been held under the escheator’s own authority, the documents were returned only into the Exchequer. Exchequer records are held by the UK National Archives in series E149 (Series I, Henry III to Richard III) - E150 (Series II, and other Inquisitions, Henry VII to Elizabeth I).

• After 1540, another copy was sent into the Court of Wards and Liveries. These are held by the UK National Archives in series WARD 7

Escheator’s accounts are held in series E136 (Note that the serach page for series E136 also has links to Chancery records in series C132 - C142, mentioned above).

This system, as it had developed by 1341, remained in use throughout the devastation caused by the Black Death, and seems to have been surprisingly effective and robust. Campbell & Bartley observe that after 1341:

"Thereafter the number of extant IPMs rises once more, surging to a temporal maximum in the plague year of 1349."

• Op. cit. p19

That 'temporal maximum' is hardly unexpected since deaths from the Black Death peaked in the summer of 1349.