You worded that perfectly and I totally agree.  The guys in the draft are all aged 21-24 and there are going to be some spoiled and immature players that have so much talent that you can't overlook them.  The Bills can handle some knuckleheads if they are harmless.  Marshawn Lynch is an example of a player who sounds really immature.  He is a goofball who is always laughing and his speech is ridled with slang.  He was considered somewhat of a character risk going into the draft.  The Bills noticed that he had a good understanding of a team concept and had a good work ethic.  They wisely dismissed concerns about character and drafted a great player.

Marcus Stroud is a perfect fit for the Bills tampa 2 defense.  I don't know why everyone assumes he is a huge run-stuffing DT (I'm not saying that you are assuming that).  At 306 pounds he is actually lighter than Kyle Williams and John McCargo.  Even Melvin Fowler, despite his lack of strength, weighs more than Stroud at 310 pounds.  Stroud is also 4 inches taller than McCargo and 5 inches taller than Williams.  His height makes him noticeably less balky than McCargo and Williams.  Stroud can control 2 gaps, but that is mostly because of how quick he is off the snap.  He uses his long arms to get into a guards body and establish control immediately, driving him into the backfield.  He can actually struggle to maintain leverage sometimes whichs leads to him not always being a stout DT.  If a guard gets him out of position, Stroud can get pushed around a little.

"I don't understand why exclusively fitting this scheme is of such paramount consideration."

That isn't a consideration at all.  Kawika Mitchell, who you mention, should prove that it doesn't matter what kind of player he is as long as he fits the general concept of the D enough to improve the team.  Kyle Williams isn't a fit for the tampa 2 either, but the Bills saw that he was great value.  Jabari Greer is small and not overly physical, yet he still won the CB job last season.

If you remove the word "exclusively" from that phrase it makes much more sense and the answer should be obvious.  The Bills play a tampa 2 defense and are looking for the best players in that system, regardless of their abilities in other schemes.  The Bills would draft a CB who is a 9 out of 10 in zone coverage, but only a 3 in man over a CB who is a 7 out of 10 against both zone and man.

I think there are many advantages to evaluating players based on the specific ways you plan on using them.  The biggest advantage is value.  Players like Flowers or Chevis Jackson are much better in zone than in man so they will go much later in the draft than they would if everyone played a cover 2 scheme.

Another thing I like about using the same basic scheme on every play is the team concept I believe it creates.  Players really learn their roles when they get to do the same thing on every play.  Players get to practice the same basic things every day until they get very good at them.  Practice breeds confidence and despite the Bills lack of talent and experience on defense they seemed like a very confident unit.

I like Antoine Cason, but I don't think his ability to jam WRs at the line is all that impressive.  He has some toughness and it shows in his ability to help in run defense, but he needs some work on jamming.  His problem there might be mental because he doesn't have the athletic ability to turn and recover if he is beat at the line.  That is just speculating though and most young corners struggle to jam effectively.  He should be good at it in time.

I think Al Harris is a decent comparison, but Cason has better ball skills and is more aggressive going for the pick.  Would you draft Al Harris at 11 if he had 6 or 7 picks a year instead of 2 or 3?  I certainly would, although that is somewhat of a best case scenario.

Yeah, we've all seen him play tons b/c all of LSU's games are televise nationally (so unfair btw).. he plays much faster than his 40 time, and the scouts know that too... but... it means he'll prob be knocked down to the 4th round imho...

if he gets to the 5th round, I think he would be a steal for us.

I'n gonna assume you meant Marcus Howard the 237 pound DE/LB out of Georgia.  He is very explosive and could play a role as a pass rusher on obvious pass situations and be a backup linebacker as well.  His special teams potential is through the roof.  He is a strange prospect and it is difficult to predict where he might get drafted.  Sometimes a player's draft stock can actually be hurt by that much versatility.  He could go anywhere between rounds 2 and 5.  I think he would be a nice pick in the fourth round, but I have this feeling someone is going to fall in love with him as a blitzing LB in a blitz heavy 4-3 scheme (philly, NYG, seatle) and take him in the third round.