] I had understood from various bios that Edward was next in line, as it were. I think that I read in the Shields bio that Edward was selected for the Knights because George and Cassandra thought that James, the first son, was to be the heir of the Leigh-Perrot fortune. It ultimately did not work out for James. The second son, George, was not an option due to his health and disability. So Edward, third son, was chosen for the Knights.

This reminds me of Mrs. Bennet indicating to Mr. Collins that Jane was nearly engaged already, so suggestively leading him to Elizabeth. James was overlooked since was nearly inheriting the Leigh-Perrot fortune as you say. Unfortunately, the Leigh-Perrots didn't pan out. I think that was more due to defiance by the cleptomaniac aunt than for anything more (What was her first name? I don't have a bio at hand.) The only thing fortunate for James was that he took over his childhood home (but then he must not have liked that well, since wasn't it James that eventually tore it down for something larger?)