I don't think I've ever played a TS3 game that didn't lag eventually, but I never had a problem like this before. I have a family of two sims, who share a house with three roommates (generated by the game, although I de-puddified them). All of a sudden, one of them joined the household (and became playable). I immediately quit my game without saving but it happened again, at the same time, when the roommate came home after his job and got promoted. Nothing I do seems to prevent this from happening. It's not a huge problem, I decided to just keep playing and ignore that he's in my household, but it's annoying. I did try to move the dude to another household and then thought I'd ask him to become my roommate again, but the stupid thing never showed up in the pie menu. Did anyone else experience this?

I got the whole set (with plants) from G4TW and it works perfectly in my game. http://www.games4theworlddownloads.org/ts3/store-2014/april.html#bohemian_garden

It seems like the base game lighthouse is only available in the CAW files for Sunset Valley, but you can import them and use in another world. There is a guide on how to do this on modthesims, written by simsample. However, if you're not familiar with CAW, there is a similar lighthouse that came with Monte Vista that is available in buy mode. You can read about it here.
Your third alternative is downloading a substitute CC lighthouse, such as St Philip lighthouse.