I recently tried spawning king Helseth into a house and it wouldn't spawn him. Does anyone know how to fix this or is there a mod to fix the npcs you can't get to spawn?

There's probably script conditions that need to be met for him to appear.
So even when you PlaceAtPC he might not appear, because other script conditions still have him set as Disabled.

That would be my guess.

there's nothing to fix except maybe your lack of info about game mechanics.

In short

- some NPCs are moved/enabled/disabled by script according to quest progression, so changing quest progression or script would be required.

Usually best way to try cheating is moving proper quest progress (journal index), look at quest infos e.g. at http://www.uesp.net/wiki/Tribunal:King_Hlaalu_Helseth

- even with always enabled NPCs, it is often a bad idea to spawn a copy of something meant to be unique. If you really must cheat changing NPC position, do it moving (e.g. position, positioncell) the original unique NPC, not spawning a copy of the NPC

That is correct - he is disabled by his original script until TR_Bamz >= 50 (after the Plaza attack).