First off, i'm placing a few items onto a surface but none of them appear there when i load the game.

Also, an NPC merchant i've made occassionally appears outside the tent he spawns in but sometimes spawns correctly.

Is there anything I could be doing wrong causing this?

don't place your havokable stuff right on a surface, but always a few units above. else they'll fall through (and they will fall down as far as possible to that surface as soon as their 3d loads anyway)

and your merchant likely just went outside for a smoke.

silly question but is the area under the npc navmeshed?

I've found that in two cases (still building my mod so I only have 2 npc's at this point) I needed to navmesh the area where they were to spawn or else they could appear down the hall or at the far side of a room. The Second npc didn't even spawn until I put down the navmesh.

Secondly, I assigned to them a small patrol which seemed to help as well as they need to be there at the patrol. in the second case I have my npc spawn and walk to a bench and just sit.