There are many parts to this problem, so let's start with a simple bidding sequence.
3H is preemptive
1. What is 4D?
2. What is 4NT?
3. What is 5D?
Let's start with 1. Do your negative doubles imply diamonds? Or can the doubler have 4324? If they imply diamonds, does that mean 4D can be bid just competitively with say 2146? I assume that for most players, 4D here is natural and forcing.
2. At least one grand life master thought it was blackwood (RKC). Is this the best use of this bid? Is opener so worried about overcaller bidding 5H that opener can't bid 4H followed by 4NT? I remember discussing this auction a few years back and someone suggested 4NT was minors with an emphasis on clubs (2+ card discrepancy) while 4D would be 1 card discrepancy. Maybe that information is more useful?
3. Here's where the questions about this auction came from. The bid at the table was 5D. So what is 5D? I've heard three different interpretations so far. Exclusion blackwood (RKC), diamond void slam try without being exclusion, and natural. My first response would be to not play this as natural because 4D would be natural and forcing for me. But then I got to thinking, and what if the auction was this:
Well now 4C isn't forcing, so 5C has to be natural, right? So should the two situations be considered similar or different? I still think in either case 4NT as showing a 2 card discrepancy is probably better than using it as RKC.
Thoughts are welcome in the comments section before I move on to the next part of the problems this hand generated.