I'm not a royalist, but ... one thing that strikes me from the Laura K interview is that Charles Spencer often describes past conversations as if he's providing direct quotes/a transcript of the original dialogue. He varies his intonation when conveying someone else's words, and even the BBC transcription puts quotes around them. For example, he recounts how his sister Lady Jane Fellows told him over the phone as soon as she got the word that Diana had died. In the BBC interview he recalls Jane saying "I'm afraid she's died." In the book (based on excerpt 2 published in the Daily Mail), CS writes: "And Jane, witnessing the completeness of his [her husband Robert Fellowes's] shock, relayed to me, in a soft voice: ‘That’s it… it’s curtains… Duch is dead…’ " It's minor, and these two versions aren't contradictory - except in the specific words used.
How, then, can we trust that KC literally and exactly said "rest assured, we'll forget her soon enough" and not something like "rest assured, this" (the uproar and controversy around Diana's funeral arrangements - which very likely WAS on KC's mind more than a pure, emotional grief response) "will be forgotten soon enough"? The atmosphere in the UK between her death and her funeral was extremely volatile and tense; I'm not sure CS, who was in Cape Town, would have fully appreciated what that was like in order to put KC's words in the full context. There's a possibility of mishearing in addition to misinterpreting - all made more likely by the fact that the two men never discussed this again and it's been 30 years. I'm not saying either one is lying or that my example of a possible alternative phrasing is correct, it just seems quite possible that CS's quotes may not reflect precisely what was said even if he believes they do.
As for being "giddy" in the aftermath of a death - well, the entire system of royalty (and aristocracy) situates and normalises the death of a loved one as the main way to get things you want. A better job, a higher-level title, more money, more land, a house, greater prestige, broader access - it all comes through the death of someone you love and whom your loved ones are mourning. It's no wonder if people are desensitised.