You can either enter TAR in the dream or in the flesh (some can do both). If you enter TAR in the flesh, you are there in the flesh. Slayer/Luc is in TAR in the flesh and thus entered TAR in the flesh before. If you can enter TAR in the flesh, you can naturally do it everywhere in the Real World, similar to leaving TAR at any location. From this ability, obviously given by the DO, and Slayer's apparent profession as hitman/assassin/bounty hunter for the Shadow, one can also deduce the other things, e.g. that Slayer likely killed the BA or the Gray Man.

Nothing necessarily indicates that Slayer is a Dreamwalker. I explained why it makes sense that he went back to Emond's Field, after he was shot.