I'm not too concerned with Cousins "demanding" a trade. If Cousins' agent is saying that DMC didn't demand a trade, then it seems like this was just heated words said in an argument.
There are two things that are more concerning to me: (1) DMC's continued immaturity, (2) Westphal's inability to communicate with his players.
As far as concern 1, DMC is still relatively young. Some of his immaturity may be attributable to the fact that, well, he's still a young man - and emotional young man at that! On the other hand, as these outbursts recur, it indicates that DMC is not learning to manage his emotions effectively. I am beginning to wonder if DMC is the new Ron Artest.
Concern 2, is also a big concern and it appears that the problem is extending well beyond the current DMC "trade demand' scenario. Westphal is losing the team, and may be losing the fan base. Tyreke's comments indicate that the communication and system is not reaching the players. The fan base is becoming increasingly frustrated about Westphal's odd coaching (e.g., telling Jimmer to guard Batum, the small-ball lineup with Jimmer and Isaiah, the apparent incompatability of an Evans, Thornton, Salmons line-up, etc.).
I don't think panic is ever an appropriate response (although it's sometimes tempting!). I'm frustrated over Westphal's statement. It seems that he is throwing DMC under the bus. There is no reason to release a statement like that to the media. To see this type of bickering in the media reflects very poorly on Westphal's professionalism.