Why different patterns of intercession in Numbers 16:20:24 and 16:44-47?
Answer: There were two separate punishments to fit each respective crime.
We might notice that even earlier in this chapter (16:7c), Moses states:
Numbers 16:4, 7c: "When Moses heard [Korah and 250 leaders of the congregation challenging he and Aaron], he fell on his face... You have gone far enough, you sons of Levi!”
Here, it appears that this dissention had been brewing for some time and that Korah and his "men of renown" had been plotting this coup for a while. It had all now come to fruition. Moses' response in verse 7c appears to demonstrate his recognition of this long-festering menace:
Numbers 16:8-9: "Then Moses said to Korah, 'Hear now, you sons of Levi, is it not enough for you that the God of Israel has separated you from the rest of the congregation of Israel, to bring you near to Himself, to do the service of the tabernacle of the LORD, and to stand before the congregation to minister to them?'" (emphasis added).
Moses must surely have feared both for his life and that of Aaron's during this menacing encounter. Often, it is hard for us to put our feet in Moses' shoes, one man beleaguered by an entire nation (not to mention his sister, Miriam). Korah and his followers were scheming for even more authority and power, indeed, enough to usurp that of Moses himself. Moses goes even further and rhetorically asks if they expect to take control over the Tabernacle (and, thus, probably the entire congregation):
Numbers 16:10b-11: "And are you seeking for the priesthood also? Therefore you and all your company are gathered together against the LORD; but as for Aaron, who is he that you grumble against him?”
All the while, "the obstinate congregation" also kept murmuring to themselves about Moses and Aaron. The entire affair brought about the anger of God who not only intended to destroy Korah and his cohorts, but the rest of the congregation along with them! Fortunately, Moses interceded for them and spared them from the same fate as Korah and his insurgents:
16:20-22: "Then the LORD spoke to Moses and Aaron, saying, 'Separate yourselves from among this congregation, that I may consume them instantly.” But they fell on their faces and said, 'O God, God of the spirits of all flesh, when one man sins, will You be angry with the entire congregation?'” (emphasis added).
You might say the congregation itself had "dodged a bullet" as God then brings disaster down on the rebels — to include the 250 "leaders of the congregation."
However, God has hardly forgotten about the congregations' grumblings: they are all walking on thin ice, as noted in the OP regarding the next verses. This is where the punishment diverges a bit from that we read about earlier:
16:41-42: "But on the [very!] next day all the congregation of the sons of Israel grumbled against Moses and Aaron, saying, 'You are the ones who have caused the death of the LORD’S people.' It came about, however, when the congregation had assembled against Moses and Aaron, that they turned toward the tent of meeting, and behold, the cloud covered it and the glory of the LORD appeared" (emphasis added).
This is not good. It seems the people just cannot learn their lesson!
16:44-45: "[The] LORD spoke to Moses, saying, 'Get away from among this congregation, that I may consume them instantly.'”
What the congregation fails to recognize is that Moses has been the one keeping them alive. Note that, this time, all Israel could be wiped out — not just Korah and his followers. It, therefore, makes perfect sense that Moses and Aaron might be able to save many of them, even though many others would be lost in the ensuing plague:
16:46-47: "Moses said to Aaron, 'Take your censer and put in it fire from the altar, and lay incense on it; then bring it quickly to the congregation and make atonement for them, for wrath has gone forth from the LORD, the plague has begun!' Then Aaron took it as Moses had spoken, and ran into the midst of the assembly, for behold, the plague had begun among the people. So he put on the incense and made atonement for the people."
The reason for the distinction between the two judgments is that the central figures in the first consisted of a limited number of men, perhaps just short of 300. In this latter case, however, tens — if not hundreds of thousands were in immediate peril, and Moses, through God, had the authority to at least halt it before it consumed many, many more.
There really is no distinction between these two patterns. The first is directed at a few, while the second is directed at the many, and as such, may be halted before all Israel is lost.