] Was, there, in fact, an entail on the estate that stipulated that it must go to a child of an Austen?

] I ask this now, because the whole action seems a bit strange to me. And also becaue later in life, Edward's inheritance was contested ...but we will come to that bit later.

I had the same thought Caroline. Was there something strange about it? Were they in a hurry to have a heir, or does it just appear so because we don't know the details. They can not possibly have known of their inability to have a child, could they? (The medical science had not yet reached the stage where they could predict infertility. They had microscopes, but they didn't understand what they saw in them).

It seems that the important thing was to keep the Family name living. And if a couple was childless, they just tried to find someone suitable to inherit. Preferably a relation but not necessary. But he needed to change his name.
In fact it was no coincidence that an Austen was chosen. The Knight and Austen families had a common ancestor, a John Austen. The line went back via Th.Knight's mother Jane Monk.

There is this thing about the legality of the entail. Something that concerned the Austen family for several years.
It was about a will that had stipulated that should Th.Knight I die childless the Chawton estate should go to a Hinton family.
But when MrTh.Knight I(formerly Broadnax and May) inherited, he cut off this entail, wich he obviously had a right to do. But there seems to have been made some formal mistake wich they used to question it.
The outcome was that Edward had to pay a substatial sum to settle it.

The Knights might have been aware of this problem. But I can't see how it would be connected to the adoption. Since the entail had already been cut off by Th.Knight I.

I like to end by quoting from J.A. A family Record:
In October 1794 Thomas Knight II had a stroke and died suddenly at his Hampshire estate of Chawton, at the age of 58. His estates were left to his widow for her lifetime. But he had made Edward his heir, as had always been promised. But he also provided in his Will that, should Edward's line fail, the estates should continue to descend to each of the other Austen sons in succession. For the immediate moment he also left £50 apiece to Cassandra and Jane, which was probably the largest sum either of them had so far ever possessed, as their father could allow them only £20 a year each for their personal expenses, and even this small sum was paid in quarterly instalments.