How did you come up with your formulae? The old "formula" the Live series used for "player of the game" was something like "points + 2*assists + 2*rebounds + 2*steals + 2*blocks". Something like that. The idea there is that an assist led to two points, so it should count as much as two points when comparing points and assists.
As for the other stats, I'm not sure why they did it that way, because they should probably be normalized by team shooting percentage if you ask me (i.e. a rebound is only going to get you a 45% chance at a score). Even that is flawed in that turnovers should be factored in.
Blah, those are some thoughts on the matter. Now, regarding injuries... If you already use Excel, I think you need to graduate from using dice and step into the big boys world. You should have a fixed chance of injury given to each player (you probably find something like that already in the stats). As it is, with the 12-sided die you give an 8.25% chance, which is huge! The way you have it, there is only a (11/12)^15 = 27% chance of having no new injuries for a given game if you allow for injuries to IR players (i.e. in practice, if the guys on IR are not hurt) and (11/12)^12 = 35% for only non-IR guys. I haven't looked at the NBA stats in depth, but I imagine that for at least half the games there are no new injuries on a given team.
Now, suppose your little Excel things tells you "yes, injury". Another way to compute the length of the injury is to use some cool math. Use excel to generate a so-called "exponential distribution". This will weight short injuries more heavily than long ones. For example, you could have the injury duration be
=CEILING(-5*LOG(RAND()),1)
You can play with that "5" as a parameter; I chose it because it looked like it might give decent numbers.
I suppose a better simulation would have some kind of weighting of real injuries (e.g. 25% of ijuries are sprained ankles) and a distribution for each such injury (50% chance ankle was only tweaked, 25% chance it's a three-week sprain, 25% it's a six-week sprain). Then you could also have guys playing hurt by modifying stats because of injuries... dang give me a job at EA, I want to work on this stuff!
Anyone bored yet?