Terrible safety.

So basically, how are Young/Hall viewed by the fans and the organization? Are they seen as the potential future or are they stopgaps until a CJ comes along?

1.  I'll preface this by saying I will defer to the pros anytime.  But in this case, I think the point that Matt is making is that CJ isn't coming from a ZB scheme.  That said, ECU definitely had ZB elements to their game, and regardless, it is a moot point.  CJ has vision, one-cut agility and patience, and that is all Denver is looking for as far as the system is concerned.  They have successfully been teaching the rest of the system to backs who had a lot less going for them than CJ does, and Shanahan can and has adjusted his offense to capitalize on what specific players bring to the table.  Portis is a perfect example.  Tatum Bell is another, less desirable, example, since with Tatum it didn't work out.  The patience factor couldn't be overlooked and Shanahan learned that one the hard way.  And while I would hesitate to call most any player in the draft "sure-fire", I would note that Denver has rarely taken anything like a "sure-fire" RB, yet it has worked out every time before.
2.  Regarding weight, size, build, style:  it really doesn't matter.  There are lots of ways to skin a cat, all that denver asks for is a one-cut runner.  Certain builds and whatnot might help or hinder, but I think the playcalling evens things out.  We all look at TD as the ideal ZB back, because of his tremendous success, but I firmly believe that replicating his build and style isn't the key to establishing another franchise running game in Denver.  We need players with CJ's attitude who are willing to work their way to the top.
3.  To answer cynyc specifically:  CJ isn't worth it because he is an upgrade over Andre Hall or even Selvin Young, nor because he has great traits for our scheme.  Lots of backs have great traits for our scheme, and except for durability issues, most Broncos fans really want to see what Young/Hall can do in just their second year.  CJ is worth it because he is an exponential upgrade of our return game, which wasn't just bad these last few years, it was the WORST.  More on this point later.
4.  CJ can add about 10 lbs without affecting his speed, which he should do for durability reasons so that he can have a long, productive career (for a RB at any rate).  If it was just for the scheme Guru would have taken Mendenhall (which he was this close to doing).  Worst case scenario his godly speed is reduced to elite among men.  No harm done.:)
5.  I get it, you don't like Marshall.  Broncos fans have stars in our eyes, but we also know what we are looking at here:  if he goes through next season as stupidly as he has gone through his offseason, Broncos fans will be right there with you.  But frankly, we're too far into this to back out now.  Regarding taking CJ, the only WRs worth considering do not possess anything that puts them head and shoulders above him.  Bennett in my eyes is the closest, and a case could be made for Jordy, but again, both project more to possession/slot type guys, and both would be as much of a reach as CJ.  I have advocated taking Bennett with #42 for a long time, and I think it can definitely help, but we have a chance to address one of the three most critical needs on this team with CJ, whereas Bennett is more of an insurance pick.  When the dam is leaking this bad, you got to plug the holes before you can paint it.
6.  Denver would change the scheme to fit a player, though, as you say, not to the degree of fundamentally changing it, i.e. taking away the playaction.  But we aren't worried about throwing to him out of the backfield.  Young got a lot of reps lining up in 4 WR sets, Johnson would get even more, but all the same you sound like a Colts fan:  throw the ball to the slot, throw the ball to WR, throw the ball to the TE, or the OL/FB,  throw the ball to the RB.  No, no, no.  Hand the ball to the running back, hand the ball to the RB, hand the ball to the RB.  :)
7.  This is the crux of it right here:  ST is the least of our 3 most important concerns!  In order of importance we need to: score in the redzone, stop the run, and fix the return game.  CJ directly addresses one of these areas and indirectly addresses another, though I think it would take some time to get him involved on offense.  And to be fair to our defense, they were playing waaaay outside  of their comfort zone last year trying to adapt to Bates' run-contain defense.  Just switching back to the zone-blitz should help tremendously.  Other than that we had to address LB (done in FA with Boss and Niko) and DT.  A draft day trade is in place for DT Dewayne Robertson (a second day pick, maybe two) and we need another solid 2 gapper, but at a fair price.  We utilize a rotation so heavily that some of the guys who would be studs elsewhere would rarely see the field.  Sims is an example.  He is a big run clogger with pass rushing moves and tons of potential, but he is far from a sure thing.  Addressing scoring in the redzone is a matter of henry being healthy (who knows until we know), trying to upgrade the line (very deep draft for that task), and improving our starting field position.  We had a lot of 70 and 80 yard drives last year that sputtered when we got to the redzone, because our backs couldn't stay on their feet.  Less touches will help and a better rotation will help.  CJ helps on both counts.