Question 1:
Many receivers do this, or their older models did this. Basically, it is assumed that the distance from mains to listener and the distance from the centre to the listener is the same when the centre delay is set at 0ms. This is due to the fact that, in general, the centre is closer to the listener than the mains. Thus, all the delay settings are relative to the mains. The only case this does not handle is if the centre is further from the listener than the mains. I am unsure, however if even the new units which have delay settings for the mains handle this scenario - perhaps they just though any excess delay for the mains away (Stupid I know, but certainly plausible given some of the other things we have seen in the past). Perhaps someone will have to test this more thoroughly (I have some ideas
Question 2:
This, also I'm not sure about. My NAD T752 has the delay settings in a separate menu item, and doesn't specifically state whether or not they are applied in DTS mode. Certainly the channel level settings and bass management settings are applied the same in DTS as in DD - I have made measurements of this to verify it. I guess I'll have to do some more measurements of the delay behaviour - (Not too hard I think).
Hope this has been some help. Actual H/K owners may be able to shed some more light on it.