First invisible weapon was an npc. I've deleted it and will post the fix soon. The 2nd issue I'm not sure. They seem to be a part of the boat to Northrend, but as it leaves they're still npcs on the boat, but once the boat leaves, these NPCs go through the boat, yet the npcs are still on the boat so I'm not entirely sure why. I also looked to see if there were multiple spawns, but they're only one per npc. Not sure if this has to do with the boat mechanics or what.
when you tag the gm on ( .gm on ) you can easily target the sword. By fear, i only moved it behind some crates in front of you in your picture (next to the stormwind intendant). But you can delete it (i don't know if the sword is linked to something else might break by delete it)
I'm trying with GM ON. But if i target it as an item, it only picks some chairs nearby. And if i try to target it as an NPC, it targets other NPCs nearby. I don't know what different command to use...
.gm on => then you can select/target the floating sword. (if you click on it...)
.npc move (or .npc del) => to move the selected/targeted sword to your current position. And if you choose the deletion, then the sword is deleted.
The sword might be a part of some event or trigger, and the creature might just have an invisible display ID. It might be best to leave it or move it somewhere until we know what its use is.