***** NOT A TEACHER *****
(1) Thank you for continued interest.
(2) What a coincidence. I was just reading a presumably authoritative book on the Web. He says that in "I was a fool to go," the infinitive is an adverbial objective. Usually, that term means a noun that is being used as an adverb. He then gives this paraphrase: I was a fool IN RESPECT OF GOING. According to what little I know, "in respect of" is, as you suggest, a preposition. In other words, if we were to analyze the paraphrase, it seems that "to go" is the object of the preposition and the whole prepositional phrase modifies "fool." But if we stick with "I was a fool to go," I see only two choices: "to go" modifies verb or it modifies the whole sentence. I guess I have to come to the conclusion that it does not modify "fool." I guess I have to surrender and accept what most people say (including you and Corum) that "I was a fool to go" = I was a fool because I went."
(3) One authority did call this construction "peculiar." And, of course, hardly anyone cares about such a minor construction.
(4) Thanks again to you and Corum for your help. I really learned a lot.