(Not too fussed about certification numbers in this forum as the gist of the thing is of more use to us as pilots.)
(a) Vmca is usually a single declared figure. I guess that there is no reason why a manufacturer can't schedule different values for different circumstances but this is not the usual approach.
While the "real" day to day Vmca clearly varies (mainly below the declared figure) this is neither quantified nor of much interest to us generally. If one happens to be in circumstances where such matters are critically significant, then one is in more strife than Speed Gordon. The AFM data is based on the certification value. Thus the Vr lower limit as specified in the AFM will not vary from day to day.
(b) V2min relates to Vmca and Vs1. The usual presentation is for V2min to reduce with reducing AUW unless the Vmca consideration becomes limiting at lower weights (mainly on 4 engined aircraft). In the latter case, V2min then becomes constant, reflecting the declared Vmca.
Normally V2 schedules in excess of the V2min selected by the manufacturer are for improved second segment climb gradient reasons.
I don't have my AFM library with me at the moment, but I can't recall any presentation where V2 is predicated on density height ?
Perhaps you could indicate an aircraft for which such variation is the case ?