Some graves don't have ghosts attached to them. I've had sims steal and return those before without any problems, though the tombstone doesn't reappear at the cemetary. As for swiping ghost graves, I don't know what happens with those, though if returning items deletes them from the world, whatever ghost was attached is probably lost now.

If you don't use NRaas mods, you might want to try running Overwatch as a one-off to see if it fixes anything. Lag has so many possible causes that it's hard to pin responsibility on any one thing that may have happened.

Are any of the worlds worth actually buying?

If you're looking for items, many of them are, but actually playing I don't know.

Can you ask other Sims to get into hot air balloon? I tried clicking at other Sims and the HBA itself... I can't ask them in!

My favorite, which has been the most "playable", is Lucky Palms. Pretty, nicely crafted, nice dock houses (which I believe were the basis for designing the houseboats coming in IP), great water effects, and very few routing issues. If only all of them were made so well. Otherwise, in all honesty, the rest pretty much suck.

Well, MV is not bad, but the architecture is so distinct, you'll have to rebuild any specific EP lots/venues you might want in the world to make them blend in, and there are few open lots for expansion (unlike LP, which left lots of room for venues and additional housing).

What about Lunar Lakes?

Lunar Lakes is too thematic if you ask me. I love Lucky Palms as well. It's a perfect world for me.