How come my follower won't enter my house even though I've navmeshed both the exterior (right up to the front door) and the interior (right up to the door as well)?

Have you finalized the navmesh? This creates a green 'teleport triangle' which tells the NPC's that there's a door there.

did you check it and finalize it ?
Do you have green triangles under your teleport markers at the door ?

[Ninjaaaaad]

Ah, that did the trick. Thanks lads!