I understand your confusion. Things can appear very contradictory. But how would you think about the following story? (My example is perhaps out of date, but it could probably give you some ideas.)
1. A teenager wanted to buy a game with the small money he had.
2. His father showed him a more sophisticated gaming machinery, in which he was much more interested, and guaranteed that he would obtain it if he would keep that money and continued assisting with their family business.
3. As time passed, the son grew up, having an amount of money in hand which was sufficient to buy a car.
4. Ridiculed by his friends for desiring for a gaming machine which is 'not suitable for an adult,' the son agreed and then decided to buy a car instead.
In this case, do you think that the father lied in guaranteeing his son that he would obtain the gaming machinery? I think that it is not the case. It was just the son himself changed his mind, even though he could obtain what he was promised.
In the same way, if Nanda continued on his ordination, he certainly would have to practise according to the discipline and the dhamma. Such is meritorious and contributes to sufficient merit (puñña) for him to be reborn as a 'devaputta' with celestial nymph retinue after death, if his mind still desired so. However, as he continued his practice and ridiculed by his fellow monks for such an inappropriate desire. He agreed, abandoned such desire and, instead, pursue a higher (highest) goal, i.e., nibbāna.
As for the case of Aṅgulimāla and Ajātasattu, it is the work of mixed kamma that complecates things. Everyone has done miscellaneous kammas. But it is the strongest vipāka that takes priority.
Will explain more when I have time, and if it is still needed. But in brief, Ajātasattu's bad kamma is like a horse, while his good kamma is like a dog. The horse runs faster. This is opposite in the case of Aṅgulimāla who was ordained and properly practised himself under the Buddha's guidance. You may find further information that his monk's life was not at all easy. But he tolerated it and trained himself appropriately, and finally won.