You agree at least that it's possible to deduce before LoC that Slayer is a deadly assassin/bounty hunter/hitman of the Shadow and uses TAR in the flesh? And you also agree that it's possible to deduce that Slayer and Luc are the same being? (e.g. both have the inhuman smell and suffer from Perrin's shot)

If you agree with this, your only problem seems to have been, how Slayer can use TAR in the flesh. You apparently (wrongly) thought that Luc is some kind of Dreamwalker, but instead of him entering TAR in the dream, the dead Isam is then somehow in TAR " in the flesh" (although he is dead..), like a Hero of the Horn, or something like that. Irrespective of the question if your idea makes any sense (in my view, it doesn't), I believe one could possibly also come up with the (actually correct!) idea that the DO gave LuC/Slayer the ability to enter TAR in the flesh at one point in the World and exist TAR again at any point, basically like a channeler can use TAR with a Gateway (we see that creatures of the Shadow often have channeler-like abilities). In this case Slayer could of course also use TAR to travel distances in the Real World, which would e.g. better explain how can Slayer quickly reach the Blight to get more Shadowspawn and quickly return to the Two Rivers or how he finds his victims. Naturally to have such an ability would make sense for someone, who was precisely created to make assassinations, namely to "course and kill". It would also explain the strange assassinations in the Stone and the WT, which apparently weren't done by a normal Gray Man, but by a special assassin.