Is the ruling to fast 16 days instead of 8 days, along with paying expiation for each day, correct, or is there another ruling, and is it necessary to pay expiation now?

The more cautious approach is that the fidyah (expiatory payment) should be half a sa' (approximately 1.5 kg) for each day whose qada' (making up) was delayed without a valid excuse. This is the view of the majority of scholars, including Malik, Shafi'i, and Ahmad, whose basis is the fatwa of Abu Hurayrah and Ibn Abbas on this matter. Abu Hanifa and a group of scholars are of the opinion that the fidyah is not obligatory. The first opinion is the more preponderant one. The fidyah is neither waived nor multiplied, even if the years of delay are numerous. As for the extra days you fasted, they are supererogatory acts, and their reward will not be lost.