Conclusion III combined with Conclusion IV says to me (let me bounce this off you - I'm hoping for further elaboration) that, as altitude increases, stall speed increases (no surprise) and that corner speed also increases (since typically occurring at stall). You indicate it increases beyond top - but is this just another way of saying that, with decreased rho and a given ClMax, V required to produce sufficent lift to maintain a sustained turn increases to the point that power required ((T-D)*V)exceeds power available? I note that the air-breathing piston engine, even with forced induction, will have diminishing power with altitude.
You have to be carefull here, because when we are discussing corner velocity, it isn't related to sustained turning theory as described by the equation for excess power cited above. Maximum rate turns at corner velocity are not sustainable, and you can see that more clearly by looking at the diagram below. It shows you where in the envelope sustained turns are possible, and everywhere else, only instantaneous turns can be achieved. Infact, if you are at the peak of that diagram, and therefore performing a maximum rate turn at corner velocity, you will also be bleeding energy at the maximum rate, that is you will have the maximum negative excess power as described by the equation you posted.
If you want to visualise the effect of altitude on corner velocity and top speed, take a look at the following two diagrams. I've included two speed scales, one showing miles per hour in true airspeed, because those are the values you normally see quoted, and the other showing knots indicated airspeed, because those are the values shown in the report, and are the values that pilots need to know, as HiTech pointed out.
Notice that these diagrams confirm what HiTech said about the corner velocity and the stall speed... They occur at the same indicated airspeed (KIAS) in both diagrams.
However, you will notice that at the higher altitude, the corner velocity gets closer to top speed, and that trend continues and is more pronounced for aircraft that don't have such a high critical altitude as the P-51 to the extent that some aircraft in that situation can not generate enough lift at their top speed to reach 6G. In that case, their envelope is defined by their top speed and the lift limit. Then they have a corner velocity defined by the point where those two lines intersect.
However, I should point out that EM diagrams produced by the military often use a placard limit and not the top speed to define the right hand side of the envelope, after all fighters often go much faster than their top speed.
Hope that helps...
Badboy