You’re right on both counts about that sentence. The article “a” is indeed missing in the clause “the disability applies to the ‘person’ as whole,” which should be corrected to read ““the disability applies to the ‘person’ as a whole.” This is a proofreading error that I wasn’t able to catch.
Then, as you correctly pointed out, in the clause “the person can’t fend off for himself or herself,” the correct phrasal verb is “fend” without the preposition “off,” so that clause should read as follows: “the person can’t fend for himself or herself.”
The phrasal verb “fend for yourself” means “take care of yourself,” which is the correct sense for the sentence in question. On the other hand, the phrasal verb “fend off someone” means to “stop someone from coming too near you” or to “stop someone from hurting you”—senses that are not intended for that sentence.
I have incorrectly used the wrong phrasal verb here. I would like to apologize for it and I would like to thank you for making the correction.
As to the “off” in “fend off,” it is indeed functioning as a preposition that indicates “physical separation or distance from a position of contact or attachment.” Of course, as you pointed out, “off” can also function as an adverb, an adjective, and even as a verb.