I agree that you present a plausible reading.
The “all” in v. 20 indeed refers to the participants in the preceding scene. But that scene includes generic “people”, then “women” and then “young women who were kept indoors”. That scene has moving parts: women rush to the temple (why else would they leave their houses?) while the virgins are confined. So when this movement results in them “all” making supplication, it has to be in the temple by the logic of the scene; but the virgins participated in the supplication since they are a part of “all”. Next in verse 21 the unity of the scene is underscored and the priest is a part of it. So this passage, verses 18-22, describes a physical movement to the temple and toward the priest in order to pray together; yet the virgins who cannot move are still a part of it. Therefore they have been in the temple to begin with.
The issue that I see with your reading in verse 30 is this part: “the temple, which a little while before was full of fear and disturbance...”.
But “fear and disturbance” occurred, according to your vision, in the streets, and not in the temple.
“So when this movement results in them “all” making supplication, it has to be in the temple by the logic of the scene;
No movement to the temple is even suggested, so the logic of the scene is what it says and what is says is that people poured out their houses into the streets , it doesn't have to be in the temple. The phrase “kept indoors” in no way translates into “kept in the temple” by any logic.
Yet I think that equating one with other is a preconceived idea you have so that is reflected in your comments about the logic of the scene and how “it has to be in the temple”.
Let's try to stick with what story says, it says that it was Heliodorus and his crowd of bodyguards, and various ‘hanger-ons” that went to the temple. Nothing more, no temple virgins, no populace rushing in with him, just what is already described.
“But “fear and disturbance” occurred, according to your vision, in the streets, and not in the temple.”
“30. they” are “his men” in vs. 27 so indeed it was in the temple but it was on the part of those who had accompanied Heliodorus to the temple.
You may not have the clue in 3 Macc. 1:18 as “chambers” still isn't the temple.(post #137)
Per the American Bible Society: vs. 18 speaks of ‘young women rushed outside with their mothers’,. (3 Macc 1,18 - BIBLIJA.net - the Bible on the Internet
www.biblija.net/biblija.cgi?Bible...3...33...3+Macc...)
No temple virgins just young women kept at home rushing into the streets with their mothers.