Where did the King come into the plot? Was he jealous of Gilles' showy wealth, too?
This is another recent myth, put about by people who do not understand the complexities of the political situation. The King did what he usually did, and what he did when Jehanne was on trial for her life: nothing. Gilles had served his purpose. The King had established his own army and no longer had to rely on brigands and mercenaries. The likes of Gilles were redundant, and in fact Charles had started to move against the dissolute barons whose soldiery terrorised the countryside, some of whom were rebelling against him. It would have done him no harm to stand back and allow Brittany to make an example of Gilles, indicating that the impunity of the nobles was well and truly over.
It is very important to remember that the plot against Gilles de Rais was nothing to do with the King of France, or, indeed, the Catholic Church. He was tried in Brittany, which was an independent Duchy at that time, and it was the coffers of Brittany which profited from his downfall. Although Gilles' judge, Jean de Malestroit, was the Bishop of Nantes, he was also Chancellor of Brittany and was acting in that capacity. Malestroit was a lifelong ally of the English; his cousin the Duke played a cunning game of switching allegiance between France and England, but signed a treaty with the latter halfway through Gilles' trial.
Although the Duchy badly needed Gilles' money, the plot was as much political as financial, and aimed partly at Charles VII of France. If his two preeminent captains were disgraced and executed for heresy and worse, it implied that he owed his throne to witchcraft and the Devil.
Further reading -
L'évêque diabolique - ?
Why is there no single estimate of the number of victims? Was it 120? Or 800? Or some number in between?
The reason no number is specified is that no number was given in court. Charge 15 of the indictment states only that for the past fourteen years, every year, every month, every day, every night and every hour... [Gilles] took, killed, cut the throats of many children, boys and girls... This would imply a huge number, although only around forty cases are mentioned in court, and many of these are mere sketches with no names or details given. It was J-K Huysmans who first mooted 800 victims, in a novel.
Further reading -
The Beast of Extermination: a numbers game