I think I would simply put "for one" underneath "I" as a modifying prepositional phrase -- similar to "by myself" or "as an example".
The idea of an "intensifier" is interesting. That makes it almost an appositive. And yet, "myself" would require the verb to change its inflection by adding an "-s".
But then, "myself" would not really modify "I". What kind of I? -- myself I. That doesn't really work.
This may be a situation where Reed-Kellogg comes up short.
Tell me what you -- or the other authorities that you know -- think.
Please pardon me if I'm talking nonsense. I have very little knowledge about these things. But I've just thought, couldn't we say it plays a role similar to that of the word "only" in "only I like X"? I'm not saying they mean the same, I only thought they have a similar role in the sentence.
Also, the idea of "for one" being an appositive doesn't quite work because if it is truly duplicating the subject then the remaining sentence without the duplication would be "For one like X". That doesn't work.
"Sentence adverb" is a new term for me. Does it mean that an adverb modifies the whole clause instead of the simple predicate? To me, adverbs modify verbs, adjectives, or other adverbs. Furthermore, they answer one of the four questions -- how, when, where, or why. "Adverb" is a part of speech. "Modifier" is a part of a sentence. I think it is pretty important that the two regimes (morphology and syntax) do not get confused. "For one" pretty clearly is a modifier. I guess the question, then, comes down to "Which sentence part does it modify?"
On another matter, I must admit, Parser, that I am interested in who are the authorities that you consult.
Still on another matter, I think it would be worth your while, dear Parser, to find a way to diagram online. "SmartDraw 7: Suite edition" is what I use.
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For example, The Oxford Dictionary of English Grammarsays they qualify the sentence as a whole rather than any particular part. For example:
Truly, certainly, assuredly, undoubtedly, etc.
My favorite is kindly. When you say "Would you kindly move?" the adverb does not mean "in a kind manner." It means Would you be kind enough to ....[BUT: He spoke kindly. = He spoke in a kind manner.]
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