Patients' - this means more than one.
If you wrote 'patient's' you would have to add an article (the), indicating only one patient.
I suppose you could say 'a patient's' if you prefer. There still has to be an article.
I don't think it's intended to be a direct quote though, stewey. (I'm assuming that because otherwise the poster wouldn't have had to worry about the position of the apostrophe.)
No colon, no quotation marks. She has given the page number so I assume she is goint to paraphrase or summarise Welch.
She still needs a comma after 'according to', which is an introductory clause.
No, not if it's just a reference in a piece of written text as part of a paragraph - the way I suggest at 18.37 was how my uni tutor taught me. Colons are only if you are going to cite the quotation in an inset section immediately following.