Is there a way I can turn off collision detection for certain objects? I'd like to be able to create illusionary walls, floors, etc. I see some forms have different options for importing collision info from the nif, but I dont really understand what they are nor do I see a "none" option.

Check and see how they did it for the skull of corruption quest in Dawnstar.

I'm pretty sure that was done with a custom object, but I'll check.

It is, and you can't. Object mustn't have collision mesh part by default which is something you do/delete in nifscope, blender or any other model editting program. You can't turn it on and off in cs. To do so you could alternatively use walls with no collision, and have invisible collision walls on the same space that are enabled/disabled by script when you want it to.

So in theory I could open the item in Nifscope, delete the mesh and save as a new object?

Yes, but not the mesh. You have to go up to Render -> Show Hidden to see the collision part of the mesh (it'll appear as non-textured simple version of the mesh). This is what you wanna delete. Just a warning though, if the wall piece is actually a side piece with a wall and floor you'll need something more advanced than nifscope and delete just the wall part, otherwise you'll end up falling trough the floor before you get the chance to see the secret entrance (unless you want to make the entrance a secret trap for the player to fall in ).