Thanks Ruskie - I've added the start of the transcipt of the letter below - please excuse incorrect legal terms, and it basically goes on along the same lines and repeats "manors tenements goods and chattels" pretty much every single line.
As you can see from the text, though, no reason is given for the Inquisition, and while the Inquisition itself says yes, he's a lunatic and his nearest male relative is a nephew, it doesn't detail anything about who might have begun this whole thing.
"GEORGE the third by the Grace of God of the United Kingdom of Great Britain and Ireland King Defender of the ffaith to his beloved William China Hart and William Prissith Esquires, William Pugh, Henry Ffoulkes and John Thomas Gentleman Greeting, KNOW YE that we have assesses twice or more of ye to inquire by the Oath of good and lawful men of Our County of Montgomery as well within liberties as without by whom the truth of the matter may be better known whether Richard Whittington late of Aberhavesp Hall in the parish of Aberhavesp in the County of Montgomery but now of Brynderwen in the parish of LLanwnog in the said County Gentleman is a Lunatic or enjoyed lucid intervals so that he is not sufficient for the Government of himself his manors messuages lands tenements goods chattels..."
The family were considered gentlemen, and both owned and leased a range of properties in the area - I'm still tracking all the farms. This fellow, Richard, isn't my direct line, but rather a brother. I know the family lived at Aberhafesp Hall for a while so I'm sure it's the right Richard, and the house (still standing) indicated a relatively wealthy background at the time.