Is there a mod that lets you break into NPC houses? Preferably one with little-to-no chance of failure? Actually, a mod that just plain opened up NPC houses by default would be better yet. It almost seems like there's already a way to do some of this in-game, but I couldn't tell you how, if so.

If by "open" you mean look into an NPC's house, that's a setting with MC that you can activate. Not sure if it's attached to a cheats module, though.

I'd forgotten about that. Does that make the house actually accessible? The one time I'd used that feature, it didn't work, as I was using it not to get inside a private area, but to get a big lot to load faster, which obviously, didn't work, and I'm not entirely sure in retrospect why I thought it would. I guess it's because it was acting similar to a private lot.

I know there is a vanilla break in option available under certain circumstances to Sims in the Private Investigator Profession, complete with lockpicking animation. If there isn't a mod that enables that interaction universally, I can't imagine it would be too difficult to implement for anyone who knows scripting...

I can do that with NRaas HomeOpener though I've just used it to peek inside inactive houses to see what my other playable sims are doing in their homes.