What is the best method for retrieval from a horizontal space, and is it required to monitor the entrants breathing zone (or as close as possible) as done in a vertical confined space entry?

Rescues for horizontal entries are essentially the same as for vertical entries. The entrant must wear a harness attached to a retrieval line (or wristlets if a harness is infeasible). The retrieval line may be attached to a manually operated winch to facilitate the non-entry rescue or, in the case of a horizontal entry, the rescue team may be able to pull on the rope to remove the entrant — practice drills will let demonstrate whether or not this is feasible.

And, with any entry, the harness and retrieval line are not necessary if their use would not help the rescue. If the entrant could get stuck on obstructions while being removed or if the retrieval line could get tangled up with an airline hose or anything else, then it would be infeasible to use the harness and line. In this case, the rescue team must enter the space to rescue the entrant in case of an emergency.