One thing not addressed regarding red-shirting is this... a major (and arguably the most important) question to ask regarding whether a student/athlete should be red-shirted or not is this... "does this student/athlete have a better probability of graduating if he redshirts?"
The whole concept of red-shirting is to facilitate the student/athlete in graduating. If a student/athlete comes into college with 10,15, 20... 25 credits already earned from AP classes and/or early college equivalent courses in High School and has an ACT of 25 or higher --- or other such indicators that suggest that graduating in 4 years is a high probability... then red-shirting will not be needed on academic grounds --- and then roster management questions can govern whether to use a red-shirt or not. But... if a student/athlete is comes in with few (or no) prior earned credits and is a marginal student (as many, perhaps most are) then red-shirting should be the only course of action --- on academic grounds --- independent of roster management criteria.
This is how it should be done. Maximize the probability of graduation. 95%+ of these guys will never see the NFL --- and those few who do... they see it for a few years at best. Use the red-shirt system on academic grounds first and then use the red-shirt system (or not use it) on roster management grounds only if the student/athlete is already in good shape academically.