Is public denunciation of heretical collective post-prayer invocations, and rebuking the Imam and congregants for them, considered changing an evil with the tongue that earns reward, or is it an act for which its perpetrator is sinful?

Collective remembrance (dhikr) after prayers is an innovation (bid'ah), and a Muslim must beware of it and adhere strictly to the Sunnah. A Muslim must change what is wrong (munkar), while observing the legitimate guidelines for change, the most important of which is that the denunciation should not lead to a greater evil, and that it should be done with gentleness and kindness. Allah Almighty says: "Invite to the way of your Lord with wisdom and good instruction." So, what you did by denouncing is not a sin, provided no forbidden speech originated from you.