I was indeed referring to Anele and the Gradual Interview information about Lord Foul possessing everyone who was possessed in the real world.
I'll try to be more clear.
1. Jeremiah is possessed by someone (Horserite)
2. The list of known possessors: the Ravers, Lord Foul, Linden. Kastenessen and Covenant do something like that too.
3. Herem is not possessing Jeremiah (busy with Joan)
4. Linden is not possessing Jeremiah
5. Covenant is very weak as a possessor
6. If Jehannum (or in a lot more unlikely case Sheol or Kastenessen or John Q. Possessor) is possessing Jeremiah, they would need to be near Lord Foul at all times. This seems wasteful, since Ravers and such are valuable resources that can surely do something more useful than babysit Jeremiah. We know Lord Foul is capable of sacrificing maximum immediate evil for the sake of greater evil later. In a pinch he can put Jeremiah in old-fashioned chains and maybe have some minor underling use old-fashioned torture or do it himself on an idle moment.
7.
Lord Foul as a stronger possessor than any of the Ravers is much better at possessing multiple people at once, even at a distance. The Ravers seem to be capable of handling more than one of only stupid creatures. Lord Foul can possess both Joan AND Roger in the "real world" and that's with them being in diferent towns from each other and beyond a power-inhibiting world-barrier.
So I think that Lord Foul, being a very powerful possessor, is permanently possessing Jeremiah while also occasionally possessing other people like Anele. I think "whispering a few words here and there" probably isn't so intensive an activity for him that it would prevent possessing Jeremiah, a son of a major enemy, just out of malice. Also I think Jeremiah plays a significant part in his plans so it can be sensible to practice with Jeremiah.
Fatal Revenant reading spoiler:
Something is definitely wrong with Jeremiah in the chapter we know about. Esmer and the ur-viles trying to stop him and failing (or pretending to)? This Jeremiah certainly has gotten a lot of power from somewhere and is noticeably more active than he used to be.
The natural question is why Foul didn't possess anyone in the First Chronicles. I think he in fact did, according to the Gradual Interview wide spectrum definition of possession. Mhoram heard Lord Foul mocking him in his prophetic dreams. This may be similar to Linden hearing Covenant's voice in her sleep and Linden accidentally ending up in Lord Foul's mind and sharing his two visions of the future, which is what I think happened in the "I am content" scene. The First Chronicles Foul would have conceivably liked to keep tabs on what Mhoram saw of the future, since that would affect the good guy war strategy, and used the opportunity for some additional psychological warfare. Meanwhile I think Foul despised others so much he would wear his own body in Ridjeck Thome and possessing Lords or Bloodguard wouldn't be any more a viable strategy for him than it was for the Ravers.