Michael's refusal to allow her to take anything by mouth--even a communion wafer--should speak volumes about Michael's beliefs regarding Terri's abilities. No matter what you think Michael's overall motives are regarding Terri, his refusal to let her parents try to feed her makes no sense unless Michael is afraid that they might succeed. If Michael is telling the truth about Terri being unable to swallow and about Terri's wanting to die, neither he nor Terri would have anything at all to lose by letting the parents try to feed Terri.
Perhaps someone can come up with an explanation for Michael's behavior which does not contradict his being an honest person, but I haven't been able to come up with one.
She can't be fed because she can't swallow. But won't let anyone try.
Where is common sense?