There is no way this perp could have known the exact sequence of events that particular morning. He couldn't have known exactly what JR would do because the weather had changed. He couldn't have known the boy's father would only wait until 8:40am and then drive his son to school (and not check on why JR didn't make it to their house to meet); he got lucky. Opportunity and luck play into these abductions.
He may have had a plan to abduct a female child, he may have seen JR before and decided he'd like to abduct her if possible, or it may have been more random. He probably had a fantasy of abducting someone and then doing whatever else it was he did. We don't know which parts he improvised on the spot. We don't know what went according to his fantasy or plan and what didn't.
I personally think he discarded the remains in a way that he determined was easiest or best for him at that time. Why didn't he go to a dumpster? I don't know. He didn't. He tossed a black garbage bag out and no one saw him do that, he didn't carefully arrange her in the open. She was inside the bag when found by the trash collectors. It was the animal maintenance people who actually first saw her remains inside the bag. (source: Denver Post).