No, there was no great "conspiracy". Charles was under pressure to marry and couldn't marry the woman he wanted, that's true. (By the way, Camilla was not too old to have children in 1981; she was in her 30s, as was Charles.) However, there was no great plan to pick out some woman merely to have children with him. He genuinely appears to have hoped that he and Diana would grow to love each other and have a successful marriage. It couldn't work because they had nothing in common, the age gap and the different upbringings and education levels were too significant, and Diana had issues pre-dating her involvement with Charles and related to her very messy family history. The two of them simply made a huge mistake. No conspiracy involved.
There's no reason to believe, either, that some royal relative like the Queen Mother somehow orchestrated the marriage. She really doesn't seem to have been very interested in Diana at all, and her close friend, Diana's grandmother, Lady Fermoy, didn't think Diana was well-suited to marry Charles.
Mistakes happen; life is much more random than some people want to believe. This was just a set of bad decisions on both sides, made by two somewhat screwed-up people who didn't know each other nearly as well as they should have. It wasn't a conspiracy.
