The first line of your question takes it for granted that the reforms of Josiah were permanently effective. All the evidence of the complaints of Jeremiah and Ezekiel works against that assumption. As I read the history, it is much more likely that the battle of Megiddo and the death of Josiah had the effect, amongst others, of destroying the political power of the "reform" party in the Jerusalem establishment and therefore overturning the reforms.
In the interests of keeping this answer concise, the most obvious evidence of the change of attitude is the establishment's treatment of Jeremiah, and especially Jehoiakim's reaction to the prophecies which were shown to him in ch36.
In Jeremiah ch19, at the Potsherd Gate, Jeremiah complains very specifically about the continuing "Molech" sacrifices in the Hinnom valley, except that he uses the alternative title "Baal".
"Because the people have forsaken me and have profaned this place... and because they have filled this place with the blood of innocents and have built the high places of Baal to burn their sons in the fire as burnt offerings to Baal" (Jeremiah ch19 vv4-5, RSV).
For the resumption of condoned idolatry in Jereusalem, we may see the complaint about the "queen of heaven" festivities in Jeremiah ch7, and the observations in the vision of Ezekiel ch8, which include the weeping for Tammuz and the worshipping of the sun.
As for the "image of jealousy" in Ezekiel ch8 v5, I believe this to be Molech. The reasoning is indirect, so bear with me.
Firstly, it seems to have been the custom, when kings did their duty, for the king to give "judgments" to the people at the north or "Benjamin" gate of the city. This was being neglected in 2 Samuel ch15 v3 and being carried out in Jeremiah ch38 v7. In this duty, they were surely representing the Lord as the true king and judge.
Secondly, I see signs that the priests of Jeremiah's time were using the equivalent north gate of the temple court as their own "place of judgment", in imitation of what was happening at the north gate of the city. I believe that this is what was meant by the "Upper Benjamin gate" where Jeremiah was held in the stocks (Jeremiah ch20 v2) and the "New Gate" whwre he was placed "on trial" (Jeremiah ch26 v10).
On that basis, I suspect that the image planted "north of the altar gate" (that "north" again, Ezekiel ch8 v5), was intended to represent Molech as "a king sitting in judgment", in blasphemous imitation of what was happening at the north gate of the city.