My very first instinct was that the trailing commissions WERE self-employment income and subject to SE tax. It's really pretty simple. The commission earned, trailing or not, were earned by your father as a for-profit effort on his part, by the "sweat of his brow" so to speak, and fit right into the definition of self-employment income.
The Code and Reg's only provide that very narrow exception to this concept.
Tom Young
